tty: more kernel-doc for tty_ldisc

Extend and highlight tty_ldisc documentation for kernel-doc even more.

We don't need to describe tty_ldisc_ref* functions again. Instead, just
include their kernel-doc from code.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-22-jslaby@suse.cz


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