Add hint for AsyncIterator incompatible return type

[ikonst]

For issue described in #5070 and documented in #14973, add a contextual link to the docs.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ikonst]

@hauntsaninja OK on the approach? If so, I'll also add the same treatment to signature_incompatible_with_supertype

[sobolevn]

Shouldn't it instead say: "consider adding async to def lauch()"?

Because in my experience, AsyncIterators are way more common with async def rather than regular def

[ikonst]

Ehh, this is confusing :|

The 'problem' here should really be reported on this line:

  class P(Protocol):
!>    async def launch(self) -> AsyncIterator[int]:
         raise BaseException

but we don't put notices proactively on declarations that smell funny, or do we?

The error on the derived Launcher.launch gives us the excuse to make a notice. What we want to say is "listen, we must error on Launcher.launch, but Launcher.launch is probably fine, consider changing the base declaration instead".

[hauntsaninja]

Thanks! I think test case might be little less confusing if it used yield to showcase the common issue, but should all work the same