docs: standardize GstCapsFeatures, GstChildProxy documentation

docs: standardize GstCapsFeatures, GstChildProxy documentation

* Don't mention explicitly that API is MT safe, this implies that
  other API is not. GStreamer API is assumed to be MT safe, thread
  safety should only be explicitly mentioned when API is *not* MT safe

* Document virtual methods in standalone comments, in order to properly
  annotate them

* Don't repeat what annotations are stating with respect to ownership
  transfer, nullability

* Misc cleanup / typo fixes / addition of links

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/755>