MONGOID-5590 - Docs: Clarify that shard key dot limitation comes from MongoDB server

docs/reference/sharding.txt

@@ -84,7 +84,7 @@ configured in the association as the field name:
   end
 
 The shard key may also reference a field in an embedded document, by using
-the "." character to delimit the field names:
+the ``.`` (dot) character to delimit the field names:
 
 .. code-block:: ruby
 
@@ -94,9 +94,9 @@ the "." character to delimit the field names:
 
 .. note::
 
-  Because the "." character is used to delimit fields in embedded documents,
-  Mongoid does not currently support shard key fields that themselves
-  literally contain the "." character.
+  Because the ``.`` character in shard keys is reserved for referencing embedded document fields,
+  MongoDB `does not support <https://www.mongodb.com/docs/manual/core/dot-dollar-considerations/>`_
+  using fields whose names contain the ``.`` character as shard keys.
 
 .. note::