Doc: fix some unused/undeclared symbols

https://bugzilla.gnome.org/show_bug.cgi?id=728812
GrlPlsFilterFunc: Document how the refcounting works

https://bugzilla.gnome.org/show_bug.cgi?id=728812
grl_pls_browse: Document that it can take NULL filter func

https://bugzilla.gnome.org/show_bug.cgi?id=728812