Document toolstate rules

[RalfJung]

I am not aware of any complete documentation around the toolstate rules. That would include things like (some are open questions from my side):

• Which tools have to be in which state to get a new nightly? How (if at all) does that relate to the status_check script that got changed here?
• A PR that changed a tool fails on bors if the tool is not "test pass". Is that correct?
• During the "beta week" (whenever exactly that is), a PR that regresses any tool fails on bors. Correct for all tools, or are there exceptions?
• CI triggers: If the last commit of a PR has message "update miri" (or probably other strings), even pre-bors CI already runs the tool tests. But, from what I saw, it does not fail if the tool tests fail. (Can we make it fail if any of the tools that got changed in the PR is not "test pass", similar to what -- I think -- the main bors CI run does?)

Are there more things I missed that people should be aware of?

Cc @kennytm