Update the rendering: fix ordering of documents

[Gregory Terzian]

Follow-up to #31505

See step two of spec, and https://github.com/servo/servo/pull/31505/files#diff-2c5948584719622eef023c56cdf90e8278971f2f2eda26e7fd0485a24c32be3cR1735

I've tried doing this in fa48454, but this resulted in various unexpected test results(which I didn't note, sorry), and so it will require some further investigation. Note that our current implementation does not order docs in any way, since it usually simply handles a message immediately when received by the script-thread.