Document ErrorKind for create_new failed due to file already exists

[anatawa12]

Location

https://doc.rust-lang.org/std/fs/struct.OpenOptions.html#method.create_new

Summary

With create_new option, we can create files if not exists and fail to create if already exists or fail with other general errors such as permission errors.

I want to retry with another file name if the failure is because file is already exists but there are no documentation about error kind in create_new.
I think the error kind for create_new failure due to file already exists is AlreadyExists.
I think it's better to document that in create_new.

[Noratrieb]

On Unix-like targets, it uses O_EXCL, which is documented to error with EEXIST (AlreadyExists). On Windows, we use CreateFileW with CREATE_NEW, which is documented to error with ERROR_FILE_EXISTS (AlreadyExists).
So this seems to be correct (everything else would also a bug, arguably).