CBIDXT-220: document projector protocol.

Change-Id: If408d9603674c8d3236a3694213ee5ee69b4f624
Reviewed-on: http://review.couchbase.org/42560
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>