ceph: add a note explaining session reject error string

error_string key in the metadata map of MClientSession message
is intended for humans, but unfortunately became part of the on-wire
format with the introduction of recover_session=clean mode in commit
131d7eb4 ("ceph: auto reconnect after blacklisted").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>