http://docs.mongodb.org/manual/core/sharded-cluster-internals/#shard-key-indexes
http://docs.mongodb.org/manual/core/sharded-clusters/

This note about multikey is confusing and not to the point about arrays.

Also, I see no mention of shard key mutability, which is none. And what to do if you need to change a shard key (insert new, remove old or reversed).

We need a section with these key words to match search engine queries for "shard key restriction" and similar searches people do when things don't work.