Explanatory paragraph on live captioning methods

Dear colleagues,
As agreed at yesterday’s meeting, John drafted a paragraph describing techniques used for captioning of live content. See issue #231:
https://github.com/w3c/apa/issues/231/

I have inserted it (with a slight modification) into the Synchronization Accessibility User Requirements, and altered the following paragraph for clarity. The Pull Request is at
https://github.com/w3c/apa/pull/232

and a rendered version is at
https://raw.githack.com/w3c/apa/saur-live-captions/saur/

Please review.

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