Bug 1649386 - Add a bit of documentation to scene building. r=gw
Bug 1649386 - Add a bit of documentation to scene building. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D81663

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/0c4662217a8d30dcbd59d17b5db39ecda2ef2926

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender