tty: add kernel-doc for tty_ldisc_ops

tty: add kernel-doc for tty_ldisc_ops

tty_ldisc_ops structure was already partially documented in a standalone
comment in the header beginning.

Move it right before the structure and reformat it so it complies to
kernel-doc. That way, we can include it in Documentation/ later in this
series.

And add the documentation for the members where missing too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-7-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>