DOCS-14409 Clarify multikey index usage (#773) (#916)

* Updating multikey sort fact; added glossary entry for blocking sort

* Bug fixes

* Bug fixes

* Bug fixes

* Incorporating internal review comments

* Removing example

* tweaks

* Resolving internal review comments

* Incorporating review comments

Co-authored-by: Ashley Brown <[email protected]>

Co-authored-by: Ashley Brown <[email protected]>

Author: mdb-ashley

source/includes/fact-multikey-index-sort-limitation.rst

@@ -1,9 +1,10 @@
 As a result of changes to sorting behavior on array fields in MongoDB
-3.6, when sorting on an array indexed with a
-:doc:`multikey index </core/index-multikey/>` the query plan includes
-a blocking SORT stage. The new sorting behavior may negatively impact
-performance.
+4.4, when you sort on an array indexed with a
+:ref:`multikey index <index-type-multikey>`, the query plan includes
+a :term:`blocking sort` stage, unless:
 
-In a blocking SORT, all input must be consumed by the sort step before
-it can produce output. In a non-blocking, or *indexed* sort, the
-sort step scans the index to produce results in the requested order.
+- The index :ref:`boundaries <multikey-index-bounds-intersecting>` for all sort 
+  fields are ``[MinKey, MaxKey]``, *and*
+
+- No boundaries for any multikey-indexed field have the same path prefix as the sort 
+  pattern.

source/reference/glossary.txt

@@ -82,6 +82,25 @@ Glossary
       maintenance operations on a sharded cluster. See
       :ref:`sharding-balancing`.
 
+   backup cursor
+      A :term:`tailable cursor` that points to a list of backup files. 
+      Backup cursors are for internal use only.
+
+   blocking sort
+      A sort that must be performed in memory before output is returned. 
+      Blocking sorts may impact performance for large data sets. Use an 
+      :term:`indexed sort` to avoid a blocking sort.
+
+      See :ref:`sort-index-use` for more information on blocking sort
+      operations.
+      
+   bounded collection scan
+      A plan used by the :doc:`query optimizer </core/query-plans>` that
+      eliminates documents with specific field value ranges. For
+      example, if a range of date field values is outside of a specified
+      date range, the documents in that range are eliminated from the
+      query plan. See :ref:`explain-output-collection-scan`.
+
    BSON
       A serialization format used to store :term:`documents <document>` and make
       remote procedure calls in MongoDB. "BSON" is a portmanteau of the words
@@ -418,6 +437,12 @@ Glossary
      The international date format used by :binary:`~bin.mongo`
      to display dates. The format is: ``YYYY-MM-DD HH:MM.SS.millis``.
 
+   indexed sort
+      A sort in which an index provides the sorted result. Sort operations that 
+      use an index often have better performance than a :term:`blocking sort`. 
+      See :ref:`Use Indexed to Sort Query Results <sorting-with-indexes>` for 
+      more information.
+
    interrupt point
      A point in an operation's lifecycle when it can
      safely abort. MongoDB only terminates an operation