Add undocumented sound functions to the documentation

[Former user]

There are several sound functions that aren't in the documentation. It'd be helpful if these could be added to the in-app docs.

fadeTo
panTo
pitchTo
rewind
stop
stopActions
stopFade
stopPan
stopPitch

Discussion is here:

https://codea.io/talk/discussion/5310/hidden-sound-functions#latest

[Oliver Dew]

I didn't mean to create this anonymously, didn't realise I wasn't signed in.