(DOCSP-21787): Cleanup for partial indexes (#982)

* (DOCSP-21787): Cleanup for partial indexes

* updates per review comments

* clarification

Author: jeff-allen-mongo

source/core/index-partial.txt

@@ -96,16 +96,15 @@ index would return an incomplete result set.
 
    db.restaurants.find( { cuisine: "Italian" } )
 
-Comparison with the ``sparse`` Index
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Comparison with Sparse Indexes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Partial indexes are be preferred over :ref:`sparse indexes
+<index-type-sparse>`. Partial indexes provide the following benefits:
 
-.. tip::
-   Partial indexes represent a superset of the functionality offered by
-   sparse indexes and should be preferred over sparse indexes.
+- Greater control over which documents are indexed.
 
-Partial indexes offer a more expressive mechanism than
-:doc:`/core/index-sparse` indexes to specify which documents are
-indexed.
+- A superset of the functionality offered by sparse indexes.
 
 Sparse indexes select documents to index *solely* based on the
 existence of the indexed field, or for compound indexes, the existence
@@ -284,4 +283,3 @@ greater than or equal to 21.
       { username: "amanda" },
       { username: "rajiv", age: null }
    ] )
-