doc: Updates and fixes

- Ignore all the *-priv.h headers
- Update grilo-sections.txt with new symbols
- Fix referecens to old GrlMediaSource and GrlMetadataSource to point to
  GrlSource
- Standardize element-type annotation to use the C symbol names
- Misc cleanups to fix gtk-doc warnings

https://bugzilla.gnome.org/show_bug.cgi?id=706487