Clarify that encoding.codecPayloadType can be unset

[ibc]

In RtpSender.send():

For each value of i from 0 to the number of encodings, check whether withParameters.encodings[i].codecPayloadType corresponds to a value of withParameters.codecs[j].payloadType where j goes from 0 to the number of codecs.

Must clarify that codecPayloadType may be unset (for the first encoding, and it that case the first media codec in codecs will be used).