Auto merge of #53404 - oconnor663:current_dir_behavior, r=alexcrichton

document the platform-specific behavior of Command::current_dir

See also https://github.com/rust-lang/rust/issues/37868.

Here's my initial wording:

> Note that if the program path is relative (e.g. `"./script.sh"`), the interaction between that path and `current_dir` varies across platforms. Windows currently ignores `current_dir` when locating the program, but Unix-like systems interpret the program path relative to `current_dir`. These implementation details aren't considered stable, and it's recommended to call `canonicalize` to get an absolute program path instead of using relative paths and `current_dir` together.

I'd like to get feedback on:

- _Should_ we consider those details stable? It might be disruptive to change them, regardless of what I can get away with claiming in docs :)
- Is `canonicalize` an appropriate recommendation? As discussed in #37868 above, there are reasons it's not called automatically in the `Command` implementation.