gfile: Document a few nullable vfuncs
gfile: Document a few nullable vfuncs

`GFile` always checks whether these vfuncs are `NULL` before calling
them, so document that it’s safe for implementations of `GFile` to not
implement them.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib