Clarify shard key index cannot be a partial index. (#5834) (#5851)

* Clarify shard key index cannot be a partial index.

* slight refactoring

* Update source/includes/limits-sharding-index-type.rst



* review changes

* final internal review changes

* add link to partial index

---------

Co-authored-by: Jeff Allen <jeffrey.allen@10gen.com>

Author: MongoCaleb

source/includes/limits-sharding-index-type.rst

@@ -3,13 +3,15 @@ key, a compound index that starts with the shard key and specifies
 ascending order for the shard key, or a :ref:`hashed index
 <index-type-hashed>`.
 
-A :term:`shard key` index cannot be a descending index on the shard key.
-Additionally, a shard key index cannot be any of the following index
-types:
-
-- :ref:`multikey <index-type-multikey>`
-- :ref:`text <index-type-text>`
-- :ref:`geospatial <index-feature-geospatial>`
-- :ref:`wildcard <wildcard-index-core>`
+A :term:`shard key` index *cannot* be:
+
+- A descending index on the shard key
+- A :ref:`partial index <index-type-partial>` 
+- Any of the following index types: 
+
+  - :ref:`Geospatial <index-feature-geospatial>`
+  - :ref:`Multikey <index-type-multikey>`
+  - :ref:`Text <index-type-text>`
+  - :ref:`Wildcard <wildcard-index-core>`
 
 .. COMMENT seealso extracts-geospatial-index-shard-key-restriction.yaml