[DOCS] Add token graph concept docs (#53339)

Adds conceptual docs for token graphs.
These docs cover:

* How a token graph is constructed from a token stream
* How synonyms and multi-position tokens impact token graphs
* How token graphs are used during search
* Why some token filters produce invalid token graphs

Also makes the following supporting changes:
* Adds anchors to the 'Anatomy of an Analyzer' docs for cross-linking
* Adds several SVGs for token graph diagrams

Author: jrodewig

docs/reference/analysis/anatomy.asciidoc

@@ -10,6 +10,7 @@ blocks into analyzers suitable for different languages and types of text.
 Elasticsearch also exposes the individual building blocks so that they can be
 combined to define new <<analysis-custom-analyzer,`custom`>> analyzers.
 
+[[analyzer-anatomy-character-filters]]
 ==== Character filters
 
 A _character filter_ receives the original text as a stream of characters and
@@ -21,6 +22,7 @@ elements like `<b>` from the stream.
 An analyzer may have *zero or more* <<analysis-charfilters,character filters>>,
 which are applied in order.
 
+[[analyzer-anatomy-tokenizer]]
 ==== Tokenizer
 
 A _tokenizer_  receives a stream of characters, breaks it up into individual
@@ -35,6 +37,7 @@ the term represents.
 
 An analyzer must have *exactly one* <<analysis-tokenizers,tokenizer>>.
 
+[[analyzer-anatomy-token-filters]]
 ==== Token filters
 
 A _token filter_ receives the token stream and may add, remove, or change

docs/reference/analysis/concepts.asciidoc

@@ -8,6 +8,8 @@ This section explains the fundamental concepts of text analysis in {es}.
 
 * <<analyzer-anatomy>>
 * <<analysis-index-search-time>>
+* <<token-graphs>>
 
 include::anatomy.asciidoc[]
-include::index-search-time.asciidoc[]
+include::index-search-time.asciidoc[]
+include::token-graphs.asciidoc[]

docs/reference/analysis/token-graphs.asciidoc

@@ -0,0 +1,104 @@
+[[token-graphs]]
+=== Token graphs
+
+When a <<analyzer-anatomy-tokenizer,tokenizer>> converts a text into a stream of
+tokens, it also records the following:
+
+* The `position` of each token in the stream
+* The `positionLength`, the number of positions that a token spans
+
+Using these, you can create a
+https://en.wikipedia.org/wiki/Directed_acyclic_graph[directed acyclic graph],
+called a _token graph_, for a stream. In a token graph, each position represents
+a node. Each token represents an edge or arc, pointing to the next position.
+
+image::images/analysis/token-graph-qbf-ex.svg[align="center"]
+
+[[token-graphs-synonyms]]
+==== Synonyms
+
+Some <<analyzer-anatomy-token-filters,token filters>> can add new tokens, like
+synonyms, to an existing token stream. These synonyms often span the same
+positions as existing tokens.
+
+In the following graph, `quick` and its synonym `fast` both have a position of
+`0`. They span the same positions.
+
+image::images/analysis/token-graph-qbf-synonym-ex.svg[align="center"]
+
+[[token-graphs-multi-position-tokens]]
+==== Multi-position tokens
+
+Some token filters can add tokens that span multiple positions. These can
+include tokens for multi-word synonyms, such as using "atm" as a synonym for
+"automatic teller machine."
+
+However, only some token filters, known as _graph token filters_, accurately
+record the `positionLength` for multi-position tokens. This filters include:
+
+* <<analysis-synonym-graph-tokenfilter,`synonym_graph`>>
+* <<analysis-word-delimiter-graph-tokenfilter,`word_delimiter_graph`>>
+
+In the following graph, `domain name system` and its synonym, `dns`, both have a
+position of `0`. However, `dns` has a `positionLength` of `3`. Other tokens in
+the graph have a default `positionLength` of `1`.
+
+image::images/analysis/token-graph-dns-synonym-ex.svg[align="center"]
+
+[[token-graphs-token-graphs-search]]
+===== Using token graphs for search 
+
+<<analysis-index-search-time,Indexing>> ignores the `positionLength` attribute
+and does not support token graphs containing multi-position tokens.
+
+However, queries, such as the <<query-dsl-match-query,`match`>> or
+<<query-dsl-match-query-phrase,`match_phrase`>> query, can use these graphs to
+generate multiple sub-queries from a single query string.
+
+.*Example*
+[%collapsible]
+====
+
+A user runs a search for the following phrase using the `match_phrase` query:
+
+`domain name system is fragile`
+
+During <<analysis-index-search-time,search analysis>>, `dns`, a synonym for
+`domain name system`, is added to the query string's token stream. The `dns`
+token has a `positionLength` of `3`.
+
+image::images/analysis/token-graph-dns-synonym-ex.svg[align="center"]
+
+The `match_phrase` query uses this graph to generate sub-queries for the
+following phrases:
+
+[source,text]
+------
+dns is fragile
+domain name system is fragile
+------
+
+This means the query matches documents containing either `dns is fragile` _or_
+`domain name system is fragile`.
+====
+
+[[token-graphs-invalid-token-graphs]]
+===== Invalid token graphs
+
+The following token filters can add tokens that span multiple positions but
+only record a default `positionLength` of `1`:
+
+* <<analysis-synonym-tokenfilter,`synonym`>>
+* <<analysis-word-delimiter-tokenfilter,`word_delimiter`>>
+
+This means these filters will produce invalid token graphs for streams
+containing such tokens.
+
+In the following graph, `dns` is a multi-position synonym for `domain name
+system`. However, `dns` has the default `positionLength` value of `1`, resulting
+in an invalid graph.
+
+image::images/analysis/token-graph-dns-invalid-ex.svg[align="center"]
+
+Avoid using invalid token graphs for search. Invalid graphs can cause unexpected
+search results.

docs/reference/analysis/tokenfilters/synonym-graph-tokenfilter.asciidoc

@@ -8,8 +8,8 @@ The `synonym_graph` token filter allows to easily handle synonyms,
 including multi-word synonyms correctly during the analysis process.
 
 In order to properly handle multi-word synonyms this token filter
-creates a "graph token stream" during processing.  For more information
-on this topic and its various complexities, please read the
+creates a <<token-graphs,graph token stream>> during processing.  For more
+information on this topic and its various complexities, please read the
 http://blog.mikemccandless.com/2012/04/lucenes-tokenstreams-are-actually.html[Lucene's TokenStreams are actually graphs] blog post.
 
 ["NOTE",id="synonym-graph-index-note"]

docs/reference/analysis/tokenfilters/word-delimiter-graph-tokenfilter.asciidoc

@@ -440,8 +440,8 @@ that span multiple positions when any of the following parameters are `true`:
 
 However, only the `word_delimiter_graph` filter assigns multi-position tokens a
 `positionLength` attribute, which indicates the number of positions a token
-spans. This ensures the `word_delimiter_graph` filter always produces valid token
-https://en.wikipedia.org/wiki/Directed_acyclic_graph[graphs].
+spans. This ensures the `word_delimiter_graph` filter always produces valid
+<<token-graphs,token graphs>>.
 
 The `word_delimiter` filter does not assign multi-position tokens a
 `positionLength` attribute. This means it produces invalid graphs for streams