Write API documentation for the stream analyzer's decoder controls

[tterribe]

The stream analyzer added a number of decoder controls to daaladec.h (see https://git.xiph.org/?p=daala.git;a=blob;f=include/daala/daaladec.h;h=69b58684569e54aa6c2dff9dcd044cc7b5d41cc1;hb=HEAD#l38), but none of these are documented.

See the documentation for Theora's decoder controls at https://trac.xiph.org/browser/trunk/theora/include/theora/theoradec.h#L40 for an example of what information should be included.