docs: standardize GstBuffer documentation

docs: standardize GstBuffer 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

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

* Document enumeration members in standalone comments, so that their
  Since tag is accounted for by gobject-introspection

* Misc cleanup / typo fixes / addition of links

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/748>