Auto merge of #31963 - barosl:rename-doc, r=alexcrichton

Describe more platform-specific behaviors of `std::fs::rename`

I did some tests myself regarding the situation when both `from` and `to` exist, and the results were:

On Linux:

`from` | `to` | Result
---- | ---- | ----
Directory | Directory | Ok
Directory | File | Error
File | Directory | Error
File | File | Ok

On Windows:

`from` | `to` | Result
---- | ---- | ----
Directory | Directory | Error
Directory | File | Ok
File | Directory | Error
File | File | Ok

This is a bit against the official MSDN documentation, which says "(`MOVEFILE_REPLACE_EXISTING`) cannot be used if `lpNewFileName` or `lpExistingFileName` names a directory." As evidenced above, `lpExistingFileName` *can* be a directory.

I also mentioned the atomicity of the operation.

Fixes #31301.

diff --cc src/libstd/fs.rs
Simple merge