tty: move locking docs out of Returns for functions in tty.h

tty: move locking docs out of Returns for functions in tty.h

Both tty_kref_get() and tty_get_baud_rate() note about locking in their
Return kernel-doc clause. Extract this info into a separate "Locking"
paragraph -- the same as we do for other tty functions.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231121092258.9334-5-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>