Adds docs about types module

[sobolevn]

Closes #11275

[JelleZijlstra]

This isn't always true; a few types in types are useful in annotations (e.g. TracebackType, which is used in __exit__ annotations).

[sobolevn]

Oh no 😆

I was not aware of that. Any others?

[ethanhs]

I think some people use types.FunctionType? Also I think I've seen types.ModuleType a few times.

[KotlinIsland]

Various types in types are properly typed and recognized by type checkers, such as GeneratorType, MappingProxyType, EllipsisType.

[sobolevn]

@ethanhs ok, then we need a new plan. Maybe we can focus on "valid" usecases of using types? Experience from typeshed will totally help! 👍

[hauntsaninja]

I also don't know that this is a particularly common issue.

[KotlinIsland]

I also don't know that this is a particularly common issue.

From my experience it is common, people be seeing a module called types and thing that it's good for typing, but it isn't 😳😳😳😳.

[KotlinIsland]

related: #11288

[hauntsaninja]

I'm not saying confusion isn't understandable, nor that we shouldn't have documentation addressing this. Just that few users have even heard of types in the first place, so not sure this is the right page for it... For context, other material on the common issues page includes "if you don't put a type on your code, mypy won't check it", which is something nearly every user of mypy will run into.

[sobolevn]

I also don't know that this is a particularly common issue.

Citing @erictraut #11286 (comment)

Symbols within the types module are not meant to be used in type annotations within typical code. We regularly need to explain in mypy and pyright issue trackers not to use FunctionType, CoroutineType, etc. repeatedly.