tty: make tty_ldisc docs up-to-date

Some of the docs in this document were a bit obsolete. Clean it up a bit
and make it correspond to the current state.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-21-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>