sip_session: update :help message

darcs-hash:20090330150755-b2873-7223ffc5df423a661a6ebd3ac1342873f2c1c566
sip_session: rename internal variable

darcs-hash:20090330150816-b2873-9bd2ba92b378bbe3051984d2115e40ce2b4e4e4f