TTY: serial, document ignoring of uart->ops->startup error

When a user has SYS_ADMIN capabilities and uart->ops->startup returns
an error in uart_startup, we silently drop the error. We then return 0
and behave as if it didn't fail. (Not quite, since we set TTY_IO_ERROR
bit and leave ASYNC_INITIALIZED bit cleared.)

This all is to allow setserial to work with improperly configured or
unconfigured ports. User can thus set port properties and reconfigure
properly.

This patch only documents this behavior.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Russel King <linux@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>