tty: serial: serial_core, reformat kernel-doc for functions

There are many annotated functions in serial_core.c, but they do not
completely conform to the kernel-doc style. So reformat them and link
them from the Documentation.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220728061056.20799-5-jslaby@suse.cz


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