* Explicitely specify which headers aren't to be included in gtkdoc-scan
  This is essentially all the headers that are not installed and only
  for internal/local usage. This also includes the orc-generated headers.
* Remove all symbols/sections that are no longer present (due to accurately
  scanning only the headers we need).
* Add or expose sections which weren't previously exposed
* Make sure the "unified" library headers (ex: gst/video/video.h) are used
  everywhere applicable. Only use the specific headers where applicable
  (such as the GL-implementation-specific objects)
* Add all documentation which was not previously exposed in the right sections
* Update 'types' file to get as many runtime information as possible

This brings down the number of unused symbols to 15 (from over 300).