DOCS-15731 Add drop/hide note to sharding key page (#2332)

* DOCS-15731 Add drop/hide note to sharding key page

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* verbiage

* internal feedback 1

* External review feedback 1

Author: ianf-mongodb

source/core/sharding-shard-key.txt

@@ -47,8 +47,9 @@ All sharded collections **must** have an index that supports the
 - If the collection is not empty, you must create the index first
   before using :method:`sh.shardCollection()`.
 
-If you drop the last valid index for the shard key, recover by
-recreating an index on just the shard key.
+You cannot :ref:`drop <collection-drop-index>` or 
+:ref:`hide <collection-hide-index>` an index if it is the only 
+non-hidden index that supports the shard key.
 
 .. _sharding-shard-key-unique:
 

source/reference/method/db.collection.dropIndex.txt

@@ -1,3 +1,5 @@
+.. _collection-drop-index:
+
 =========================
 db.collection.dropIndex()
 =========================

source/reference/method/db.collection.hideIndex.txt

@@ -1,3 +1,5 @@
+.. _collection-hide-index:
+
 =========================
 db.collection.hideIndex()
 =========================