tty: tty_io: Fix some kernel-doc issues

tty: tty_io: Fix some kernel-doc issues

Demote non-conformant headers and supply some missing descriptions.

Fixes the following W=1 kernel build warning(s):

 drivers/tty/tty_io.c:218: warning: Function parameter or member 'file' not described in 'tty_free_file'
 drivers/tty/tty_io.c:566: warning: Function parameter or member 'exit_session' not described in '__tty_hangup'
 drivers/tty/tty_io.c:1077: warning: Function parameter or member 'tty' not described in 'tty_send_xchar'
 drivers/tty/tty_io.c:1077: warning: Function parameter or member 'ch' not described in 'tty_send_xchar'
 drivers/tty/tty_io.c:1155: warning: Function parameter or member 'file' not described in 'tty_driver_lookup_tty'
 drivers/tty/tty_io.c:1508: warning: Function parameter or member 'tty' not described in 'release_tty'
 drivers/tty/tty_io.c:1508: warning: Function parameter or member 'idx' not described in 'release_tty'
 drivers/tty/tty_io.c:2973: warning: Function parameter or member 'driver' not described in 'alloc_tty_struct'
 drivers/tty/tty_io.c:2973: warning: Function parameter or member 'idx' not described in 'alloc_tty_struct'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Nick Holloway <alfie@dcs.warwick.ac.uk>
Cc: -- <julian@uhunix.uhcc.hawaii.edu>
Cc: Marko Kohtala <Marko.Kohtala@hut.fi>
Cc: Bill Hawes <whawes@star.net>
Cc: "C. Scott Ananian" <cananian@alumni.princeton.edu>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Andrew Morton <andrewm@uow.edu.eu>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-13-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>