Review, define, and document RTKQ intended timing behavior and invariants

[markerikson]

I think one of our biggest issues atm is that we don't have actual documented specifications and intent for how RTKQ should behave with timing-related aspects. We've got code, and we've got a decent amount of tests, but there isn't anything written down that says "here's what we intend this to do".

Areas I'm thinking about:

• Timing of queries, mutations, and invalidations
• All the logic around returning promises from thunks and when those resolve
• ????

I think we need to take some time to document the current behavior, and then discuss what we want to have happen.

It would probably help to write out a number of use cases (such as some of the ones described in #2203 and #3105 ), and decide at a high level how we'd want to handle them.

We should also review the cache lifecycles behavior as well, like how they don't run if there's a cache hit (and what might make sense in that case).