Add pointers to the relevant issue for some bugs.
Add documentation for the DOM.
Merge pull request #2931 from Ms2ger/dom-docs

Add documentation for the DOM; r=jdm
Merge pull request #2932 from Ms2ger/fixme-347

Add pointers to the relevant issue for some bugs; r=jdm
Track rerender msgs

If the compositor outputs to a file:
* Track rerender msgs sent from the compositor to the render tasks.
* Before outputting, wait until all rerender msgs are processed by the
  render tasks.

Fixes issue https://github.com/servo/servo/issues/2871.
Merge pull request #2930 from bjwbell/track-rerender-msgs

Track rerender msgs