MB-11066: Document mutable config tokens

Change-Id: I2a6d03225e17e3b88a33e327e153ed4cfeb3bf27
Reviewed-on: http://review.couchbase.org/40615
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>