docs: standardize GstClock documentation

docs: standardize GstClock 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 virtual methods in standalone comments, so that parameters
  can be documented. This is not critical here, as parameters do not
  need annotations / specific documentation, but serves as an up to
  date example

* 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/756>