Add documentation about using the `webkit-test-runner` header for changing feature enabled state per-test

[twilco]

Adding <!-- webkit-test-runner [ FooFeatureEnabled=true ] --> to the top of a test allows the feature to be enabled for just that test. It would be great to add documentation for this.

Here's an example of where we use this:

https://github.com/WebKit/WebKit/blob/ce9238afa421a5eefce7c51d5c627a8b7eacaf79/LayoutTests/imported/w3c/web-platform-tests/video-rvfc/request-video-frame-callback-webrtc.https.html#L1