Update model layer docs with more strict rules around `mobx.action` and async/Promise functions

[tephenavies]

Wrapping an async function with mobx.action can hide mobx reactivity bugs. mobx.action (and flow, runInAction, untracked, etc.) should be used conservatively in places where a user causes a state mutation, or when an async request (started by a user) finishes and state must be mutated.

[tephenavies]

See #5026 and #5032 for an example of this type of problem.

[wibily]

ticket is for updating docs for contributors