Editorial issues. Fixes #45

... for each distinct clock rate used by the audio codecs it offered, as specified in ..., except for codecs that have their own internal silence suppression support. For codecs that have their own internal silence suppression support, the appropriate fmtp parameters MUST be specified indicating that silence suppression for that codec is desired. For example, when using the Opus codec, the "usedtx=1" parameter would be specified. This option allows the endpoint to significantly reduce the amount of audio bandwidth it receives, at the cost of some audio fidelity, depending on the quality of the remote VAD algorithm.