Ruby-2339 Write Driver Query Cache documentation (#2029)

* RUBY-2339 query cache documentation

* RUBY-2339 fix wording

* RUBY-2339 fix title

* RUBY-2339 style

* RUBY-2339 formatting

* RUBY-2339 PR comment fixes

* RUBY-2339 style

* RUBY-2339 fix grammar and formatting

Co-authored-by: Grace Chong <[email protected]>

Author: gjchong25

source/tutorials/ruby-driver-crud-operations.txt

@@ -130,6 +130,44 @@ object, or with ``Decimal128.from_string()``.
    price = BSON::Decimal128.from_string("428.79")
    # => BSON::Decimal128('428.79')
 
+Query Cache
+===========
+
+If the Ruby driver's query cache is enabled, it will cache find queries on
+the currently executed thread and avoid sending requests to the database for
+identical queries. The QueryCache class stores CachingCursor objects which attempt
+to load documents from memory before retrieving them from the database. Performing
+any write operations such as insert, update, or delete clears the query cache. Note that
+if the number of results is too large to be returned in a single batch,
+the query cache will not be used, even if ``Mongo::QueryCache.enabled`` is true, and
+an error will be returned.
+
+Similar to Mongoid's query cache, the driver's query cache implementation
+does not support using CachingCursors for queries with different limit sizes if
+the original query has a specified limit. For example, if a query with a limit of
+100 were executed, followed by a query with a limit of 10, the second query
+would still run against the database and a new CachingCursor object would be stored,
+rather than using the existing one. However, if a query does not specify a limit
+initially, then any query that is run after it with a specified limit will use
+the original CachingCursor rather than going back to the database, and will return
+the correct number of documents accordingly.
+
+To enable the query cache on a global scope:
+
+.. code-block:: ruby
+
+  Mongo::QueryCache.enabled = true
+
+The following code shows how to enable the query cache within the context of a
+specific block:
+
+.. code-block:: ruby
+
+  Mongo::QueryCache.cache do
+      # all queries within this block are cached
+  end
+
+
 Reading
 =======