bpo-45554: Document multiprocessing.Process.exitcode values (pythonGH-30142)

This addresses [bpo-45554]() by expanding the `exitcode` documentation to also describe what `exitcode` will be in cases of normal termination, `sys.exit()` called, and on uncaught exceptions.

Automerge-Triggered-By: GH:pitrou
(cherry picked from commit 3852269)

Co-authored-by: John Marshall <jmarshall@hey.com>

Author: jmarshall

Doc/library/multiprocessing.rst

@@ -569,8 +569,15 @@ The :mod:`multiprocessing` package mostly replicates the API of the
    .. attribute:: exitcode
 
       The child's exit code.  This will be ``None`` if the process has not yet
-      terminated.  A negative value *-N* indicates that the child was terminated
-      by signal *N*.
+      terminated.
+
+      If the child's :meth:`run` method returned normally, the exit code
+      will be 0.  If it terminated via :func:`sys.exit` with an integer
+      argument *N*, the exit code will be *N*.
+
+      If the child terminated due to an exception not caught within
+      :meth:`run`, the exit code will be 1.  If it was terminated by
+      signal *N*, the exit code will be the negative value *-N*.
 
    .. attribute:: authkey
 

Misc/ACKS

@@ -1095,6 +1095,7 @@ Vincent Marchetti
 David Marek
 Doug Marien
 Sven Marnach
+John Marshall
 Alex Martelli
 Dennis Mårtensson
 Anthony Martin