Rollup merge of #82943 - kornelski:threadstdio, r=joshtriplett

Rollup merge of #82943 - kornelski:threadstdio, r=joshtriplett

Demonstrate best practice for feeding stdin of a child processes

Documentation change.

It's possible to create a deadlock with stdin/stdout I/O on a single thread:

* the child process may fill its stdout buffer, and have to wait for the parent process to read it,
* but the parent process may be waiting until its stdin write finishes before reading the stdout.

Therefore, the parent process should use separate threads for writing and reading.

These examples are not deadlocking in practice, because they use short strings, but I think it's better to demonstrate code that works even for long writes. The problem is non-obvious and tricky to debug (it seems that even libstd has a similar issue: #45572).

This also demonstrates how to use stdio with threads: it's not obvious that `.take()` can be used to avoid fighting with the borrow checker.

I've checked that the modified examples run fine.