add description to the define

#define VARIABLE_SPEED_DECODING                         /* FhG: variable speed decoding employing the JBM functioniality */

#define JBM_TSM_ON_TCS                                  /* FhG: run the TSM part of JBM on the TCs instead of the final output pcm waveforms */

#define FIX_DTX_428

#define FIX_DTX_428                                     /* FhG: fix for issue 428, crash with DTX and bitrate switching */

/* ################## End DEVELOPMENT switches ######################### */

/* clang-format on */