xen/xenbus: document will_handle argument for xenbus_watch_path()

xen/xenbus: document will_handle argument for xenbus_watch_path()

Commit 2e85d32b1c86 ("xen/xenbus: Add 'will_handle' callback support in
xenbus_watch_path()") added will_handle argument to xenbus_watch_path()
and its wrapper, xenbus_watch_pathfmt(), but didn't document it on the
kerneldoc comments of the function.  This is causing warnings that
reported by kernel test robot.  Add the documentation to fix it.

Fixes: 2e85d32b1c86 ("xen/xenbus: Add 'will_handle' callback support in xenbus_watch_path()")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401121154.FI8jDGun-lkp@intel.com/
Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240112185903.83737-1-sj@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>