DOCS-14827: $merge and partial indexes

Author: npentrel

source/reference/operator/aggregation/merge.txt

@@ -184,6 +184,9 @@ The :pipeline:`$merge` takes a document with the following fields:
       - The unique index can be a :ref:`sparse <index-type-sparse>`
         index.
 
+      - The unique index cannot be a :ref:`partial <index-type-partial>`
+        index.
+
       - For output collections that already exist, the corresponding
         index must already exist.
 
@@ -517,10 +520,10 @@ The ``newrestaurants`` collection will contain documents with
 information on new restaurant openings by month (``date`` field) and
 postcode (shard key); specifically, the :ref:`on <merge-on>`
 identifier is ``["date", "postcode"]`` (the ordering of the fields
-does not matter). Because :pipeline:`$merge` requires a unique,
+does not matter). Because :pipeline:`$merge` requires a unique
 index with keys that correspond to the :ref:`on
 <merge-on>` identifier fields, create the unique index
-(the ordering of the fields do not matter): [#uniqueoption]_ 
+(the ordering of the fields do not matter): [#uniqueoption]_
 
 .. code-block:: javascript