Fixes part of #150524.
Recently I was studying RenderObject and related code to understand
how the rendering library tracks which objects need layout, and in
particular just how the concept of "relayout boundary" works.
The [RenderObject._relayoutBoundary] property itself had no docs,
and there were some other areas that I felt could be clearer;
so here's a PR where I add those docs.
In addition to docs, this makes some small pure refactors. Mostly
those are in order to make clear docs easier to write. One rename
is to fix a lone straggler from #4425 / #4483.
One key thing I learned, which this documents, is an invariant that
this code maintains on [_relayoutBoundary]. With that invariant in
mind, I found it's possible to simplify that bookkeeping, and to get a
measurable performance improvement as a bonus. To keep things simple,
though, I'll leave those for follow-up PRs.
For ease of review, the PR is broken into several commits each with
their own commit message.
e5a35f458f