Document the scheduler changes for monorepo in cocoon

[jtmcdole]

Documentation for presubmits

[jtmcdole]

For the "we get notified of check status" - we are already notified of completion from both LUCI and GitHub. To query the check status; there's already some code in the gitub utility file that could be reused; or we could use a document in firebase that has a TTL on it.

I'm leaning:

1. Keep ci yaml validation open longer; it retains the "block submit"
2. Make a new non-blocking "engine sentitle" for clarity; complete it with engine build status.
3. Finish ci yaml once everything has been scheduled.

@yjbanov - We could remove the blocker on CI Task as well if we use CI yaml validation as the blocker. That leaves one blocker with multiple exists, but a good crumb trail.

Thoughts?

[yjbanov]