API documentation updates.
- Provide an Overview section which describes some of the general design decisions of the libopusfile API. - Explicitly state that we always decode at 48 kHz. - Make it clearer that we always apply the header gain. - Document the automatic soft-clipping support. - Document that stream open failure does not close the source. - Other minor fixes. Thanks to tangobravo and siriusbtx for asking the questions which prompted this update.