[webrtc-pc] Does insertDTMF append to or replace the current tone buffer

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

== Does insertDTMF append to or replace the current tone buffer ==
Step 7 says that it replaces tone buffer it but the name implies otherwise so this might need some clarity to the introduction. If this changes, need to also update example 20. 

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1260 using your GitHub account