add clarification to unique _id index if not part of shard key

Author: kay-kim

source/core/sharding-shard-key.txt

@@ -91,15 +91,35 @@ recreating an index on just the shard key.
 Unique Indexes
 ~~~~~~~~~~~~~~
 
-For a sharded collection, only the ``_id`` field index and the index on
-the shard key or a :term:`compound index` where the shard key is a
-:ref:`prefix <compound-index-prefix>` can be :doc:`unique
-</core/index-unique>`:
+For a ranged sharded collection, only the following indexes can be
+:doc:`unique </core/index-unique>`:
 
-- You cannot shard a collection that has unique indexes on other fields.
+- the index on the shard key
 
-- You cannot create unique indexes on other fields for a sharded
-  collection.
+- a :term:`compound index` where the shard key is a :ref:`prefix
+  <compound-index-prefix>`
+
+- the default ``_id`` index
+
+  .. note::
+  
+     If the ``_id`` field is not part of the shard key, ``_id`` index
+     only enforces the uniqueness constraint per shard and **not**
+     across shards.
+
+     For example, consider a sharded collection (with shard key ``{x:
+     1}``) that spans two shards A and B. Because the ``_id`` key is
+     not part of the shard key, the collection could have a document
+     with ``_id`` value ``1`` in shard A and another document with
+     ``_id`` value ``1`` in shard B.
+
+The unique index constraints mean that:
+
+- For a to-be-sharded collection, you cannot shard the collection if
+  the collection has other unique indexes.
+
+- For an already-sharded collection, you cannot create unique indexes
+  on other fields.
 
 Through the use of the unique index on the shard key, MongoDB *can*
 enforce uniqueness on the shard key values. MongoDB enforces uniqueness