Document Drop behavior of ChildStd[in, out, err]

[abonander]

Piped child process I/O streams are implicitly closed on-drop (via Drop impls on the OS file descriptor wrappers), but this is not documented anywhere, giving the impression that there's no safe way to close these streams.

This is causing some confusion in the wild (https://www.reddit.com/r/rust/comments/65txlh/_/dgi86yp), so we should note this behavior in the docs for ChildStd[in, out, err].

[frewsxcv]

Opened a PR for this: #44625