The event type for instant-rate-change events was poorly chosen,
leading to them being re-sent too late and even after EOS.
Add a mechanism in GstPad for the sticky event order to be
different to the value of the event type to fix that up.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3387>
The documentation for several gst_*_writable_structure functions stated
that they would never return NULL, without making clear that the passed
object is required to be writable. This changes the wording in those
cases to make that requirement more clear.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1784>
