Warmup: Document warmup stages
Warmup: Document warmup stages

Document the indiviual warmup stages, and add a flowchart of the
possible state transitions.

Change-Id: Ic44b768fb65d7980be2f4cb5d4cd6588c64e1258
Reviewed-on: http://review.couchbase.org/105871
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine