[doc] Clarify that Popen.wait is only a busy-loop in certain cases

Documentation

In the docs for Popen.wait(), it states:

Note The function is implemented using a busy loop (non-blocking call and short sleeps). Use the asyncio module for an asynchronous wait: see asyncio.create_subprocess_exec.

This is actually only true in a specific circumstance: when both using Posix/Unix and a timeout is given. If, on the other hand, no timeout is given, then it does not go into a busy-loop but instead blocks on os.waitpid(). And for Windows, it never goes into a busy-loop, as it uses the WaitForSingleObject() windows API.

This note in the docs caused me to refactor my code to use asyncio - only later did I actually check what python did internally and discovered the note was wrong.

Start with the Popen.wait() documentation linked in the issue and compare the note with the platform and timeout behavior described here. Revise the note so it limits the busy-loop claim to POSIX/Unix waits with a timeout while accurately covering no-timeout and Windows cases. Done means the wording is technically correct and retains the asyncio guidance.