Documentation updates for the CTLs.

In addition to general reformatting and cleanup, this fixes a
 couple of important mistakes:
- The arguments to OPUS_SET_FORCE_CHANNELS are now 1 and 2, not 0
   and 1 (as they were when this was called OPUS_SET_FORCE_MONO).
- The default encoder mode is now constrained VBR, not unconstrained
   VBR.
It also documents defaults for all the other parameters.