Warn about likely incorrect .stop() invocations.

[nathanhammond]

During development it is likely possible to catch incorrect .stop() invocations. There are two scenarios I can identify that would have this problem:

1. .stop() invocations that would occur on the wrong context as they occur in the user actions instead of built-in actions. (See initialState lifecycle seems to miss "after" transition. #1403 and Document stopping actors. #1434.)
2. Invocations of stop() that happen in the same action resolution pass as the associated spawn().

Note that both of these problems would exhibit the same behavior, but would have a different cause.

I think it should be doable to warn at runtime about incorrect .stop() invocations - but we'd have to look into it. This would certainly improve the overall DX as we could both warn about this operation not being correct and we could point to this section of the documentation for further reading. Could you create an issue about it? - @Andarist, in #1434