avutil/pixfmt: improve definition of AVColorRange

avutil/pixfmt: improve definition of AVColorRange

As it was brought up that the current documentation leaves things
as specific to YCbCr only, ICtCp and RGB are now mentioned.
Additionally, the specifications on which these definitions of
narrow and full range are defined are mentioned.

This way, the documentation of AVColorRange should now match how
most people seem to read interpret it at this point, and thus
flagging RGB AVFrames as full range is valid not only according to
common sense, but also the enum definition.