tty: add kernel-doc for tty_port

tty: add kernel-doc for tty_port

tty_port used to have only short comments along its members. Convert
them into proper kernel-doc comments in front of the structure. And add
some more explanation to them where needed.

The whole structure purpose and handling is documented at the end too --
some pieces of preexisting text moved to this place.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-3-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>