protobuf message documentation.

Change-Id: Ib0e254e0d9c764dd941472e4ea352b9d725d8bb2
Reviewed-on: http://review.couchbase.org/40530
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>