[Autoplay] Update autoplay.md

Update the autoplay documentation to reference the policy.

BUG=1152627

Change-Id: I29ff788463b4db88389cb87a2c23de9d1f6580fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622662
Auto-Submit: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842253}

docs/media/autoplay.md

 # Autoplay of HTMLMediaElements
 
-Autoplay is the concept of playing media elements without user gesture. On
-desktop, autoplay is always allowed. On mobile, only muted video elements are
-allowed to autoplay. The autoplay logic follows
-the
+Autoplay is the concept of playing media elements without user gesture. The
+policy that defines when autoplay is allowed is defined [here](https://www.chromium.org/audio-video/autoplay).
+The autoplay logic follows the
 [HTML spec](https://html.spec.whatwg.org/multipage/embedded-content.html#media-elements).
 
 There are two ways of initiating autoplay: