util: Clarify documentation of wl_dispatcher_func_t

Adjust the brief, clarify the behavior and arguments, correct a grammar
error, document the parameters, and document the return type.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>