DOCS-93 and DOCS-196 sharding limitations.

Author: Sam Kleinman

source/reference/limits.txt

@@ -75,7 +75,6 @@ Limits
    MongoDB will only return sorted results on fields without an index
    *if* the sort operation uses less than 32 megabytes of memory.
 
-
 .. _limit-nested-depth:
 
 .. describe:: Nested Depth for BSON Objects
@@ -84,3 +83,27 @@ Limits
 
    MongoDB only supports 100 levels of nesting for :term:`BSON
    documents <document>`.
+
+.. _limit-sharding-limitations:
+
+.. describe:: Operations Unavailable in Sharded Environments
+
+   The :dbcommand:`group` does not work with sharding. Use
+   :dbcommand:`mapreduce` or :dbcommand:`aggregate` instead.
+
+   :func:`db.veal()` is incompatible with sharded collections. You may
+   use :func:`db.eval()` with un-sharded collections in a shard
+   cluster.
+
+   :operator:`$where` does not permit references to the ``db`` object
+   from the :operator:`$where` function. This is uncommon in
+   un-sharded collections.
+
+.. _limit-sharding-unique-indexes:
+
+.. describe:: Unique Indexes in Sharded Collections
+
+   MongoDB does not support unique indexes across shards, except when
+   the unique index contains the full shard key as a prefix of the
+   index. In these situations MongoDB will enforce uniqueness across
+   the full key, not a single field.