tty: add kernel-doc for more tty_port functions

From the main tty_port functions, only tty_port_destroy() was
documented. Document more of them, so that we can reference them in
Documentation/ later in this series.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-19-jslaby@suse.cz


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