docs: Removing GnomeVFS left bits

gnomevfs was removed time ago but there are still some left bits.

https://bugzilla.gnome.org/show_bug.cgi?id=725658
video-overlay-composition: add GST_CAPS_FEATURE_META_GST_VIDEO_OVERLAY_COMPOSITION
pango: basetextoverlay: handle video/x-raw(ANY) if downstream supports the GstVideoOverlayCompositionMeta API

https://bugzilla.gnome.org/show_bug.cgi?id=721953
tests: add textoverlay passthrough with composition feature unit tests

https://bugzilla.gnome.org/show_bug.cgi?id=721953
pango: demote debug WARNING to LOG for variable framerate video input

No need why we need to warn about that, it's perfectly allowed.

https://bugzilla.gnome.org/show_bug.cgi?id=725837