tty: fix tty_port_tty_*hangup() kernel-doc

tty: fix tty_port_tty_*hangup() kernel-doc

The commit below added a new helper, but omitted to move (and add) the
corressponding kernel-doc. Do it now.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Fixes: 2b5eac0f8c6e ("tty: introduce and use tty_port_tty_vhangup() helper")
Link: https://lore.kernel.org/all/b23d566c-09dc-7374-cc87-0ad4660e8b2e@linux.intel.com/
Reported-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20250624080641.509959-6-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>