We have a "good" implementation of g_clear_signal_handler() in
form of a macro. Use it, and don't duplicate the code.

Also add a comment to the documentation that "instance" in fact must
not point to a valid GObject instance -- if the handler ID is unset.

Also reword the documentation about the reasoning for why a macro
version exists. The reason is not to use the function "without
pointer cast". I don't think the non-macro version requires any
pointer cast, since "instance" is a void pointer. Was this referring
to the handler_id_ptr? That doesn't seem right either, because the
caller should always provide a "gulong *" pointer and nothing else.