docs: tokio::fs::create_dir_all() specify if path existing is an error

[jaskij]

As in the title. create_dir() clearly states that path existing causes an error. create_dir_all() does not specify that this is an error, but it also does not specify that it's not an error. Nor is it mentioned as a platform-specific behavior, unless we assume that mkdir references mkdir(1), not mkdir(2) or mkdir(3), which makes little sense to me.

[Darksonn]

Docs are very welcome. Perhaps we should just say that behavior is equivalent to std::fs?

[jaskij]

std::fs::create_dir_all has the exact same omission in the documentation - should I open an issue against the standard library first, so they can clarify?

[Darksonn]

Sounds good to me.