Documentation: improve line discipline method descriptions

Mention that the ldisc open method must set tty->receive_room, and
that many methods are optional. Add description of receive_buf2 method.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>