Document how convolver node works

Taken from
https://github.com/WebAudio/web-audio-api/blob/master/convolution.html,
written by Chris Rogers, and modified to use markdown instead of html.

Images included.

Bug: 961852
Change-Id: Ia8b7542b653c0c9157db3de9f76d7186f2bef79f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700386
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677483}