pythongh-87452: Improve the Popen.returncode docs

Clarifies that it remains None until a method checking the child process status has been called and noticed it has terminated.
(cherry picked from commit 68f5836)

Co-authored-by: Joshua Herman <30265+zitterbewegung@users.noreply.github.com>

Doc/library/subprocess.rst

@@ -921,9 +921,12 @@ Reassigning them to new values is unsupported:
 
 .. attribute:: Popen.returncode
 
-   The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly
-   by :meth:`communicate`).  A ``None`` value indicates that the process
-   hasn't terminated yet.
+   The child return code. Initially ``None``, :attr:`returncode` is set by
+   a call to the :meth:`poll`, :meth:`wait`, or :meth:`communicate` methods
+   if they detect that the process has terminated.
+
+   A ``None`` value indicates that the process hadn't yet terminated at the
+   time of the last method call.
 
    A negative value ``-N`` indicates that the child was terminated by signal
    ``N`` (POSIX only).