Add mutator collector coordination documentation

This affects a lot of ART and should thus be documented.

It is sufficiently low level that I believe it belongs in the source
tree. It should only be of interest to ART developers, in a broad sense
that includes anyone trying to understand failures involving ART.

Various comment improvements around thread suspension and checkpoints.

Test: Built AOSP.
Change-Id: I77338ff1d6f7c6aefb7be849449770438c15c982
6 files changed