[mediacapture-record] Address comments from TAG review

dontcallmedom has just created a new issue for 
https://github.com/w3c/mediacapture-record:

== Address comments from TAG review ==
See https://github.com/w3ctag/spec-reviews/issues/131:
> One other thought from me is that the examples could use a little 
more prose. For example, in the second example:
>  *  how does the video element relate to what's going on?
>  * what controls the object lifetimes, i.e., when does the 
MediaRecorder become GC-able?
>
> We're still interested in seeing work using ReadableStream here, 
perhaps as a return value from start().

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-record/issues/115 using your 
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