[webrtc-pc] insertDTMF must use must more

annevk has just created a new issue for 
https://github.com/w3c/webrtc-pc:

== insertDTMF must use must more ==
Per https://github.com/w3c/webrtc-pc/issues/768#issuecomment-248592749
 raising an issue for actually requiring that a-d become A-D. You 
should probably state something to the effect of "a-d must be (ASCII) 
uppercased to A-D". I guess you don't really have to define uppercase 
here since the range of code points is fairly well understood.

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/821 using your GitHub account