DOCS-956 covered index

Author: kay-kim

source/applications/indexes.txt

@@ -127,7 +127,7 @@ A covered query is a query in which:
 Because the index "covers" the query, MongoDB can both match the
 :ref:`query conditions <read-operations-query-document>` **and** return
 the results using only the index; MongoDB does not need to look at
-documents outside of the index.
+the documents, only the index, to fulfill the query.
 
 Querying *only* the index can be much faster than querying documents
 outside of the index. Index keys are typically smaller than the

source/core/indexes.txt

@@ -54,11 +54,12 @@ MongoDB indexes have the following core features:
   - all the fields returned in the documents that match the query are
     in the same index.
 
-  When an index covers a query, the database can both match the
+  When an index covers a query, the server can both match the
   :ref:`query conditions <read-operations-query-document>` **and**
   return the results using only the index; MongoDB does not need to
-  look at documents outside of the index. Querying the index can be
-  faster than querying the documents outside of the index.
+  look at the documents, only the index, to fulfill the query.
+  Querying the index can be faster than querying the documents outside
+  of the index.
 
   See :ref:`indexes-covered-queries` for more information.
 

source/reference/explain.txt

@@ -156,7 +156,9 @@ Core Explain Output
    Specifies the total number of documents scanned during the query.
    The :data:`nscannedObjects` may be lower than :data:`nscanned`, such
    as if the index :ref:`covers <indexes-covered-queries>` a query. See
-   :data:`indexOnly`.
+   :data:`indexOnly`. Additionally, the :data:`nscannedObjects` may be
+   lower than :data:`nscanned` in the case of multikey index on an
+   array field with duplicate documents.
 
 .. data:: nscanned