gh-138234: clarify returncode behavior for subprocesses created with shell=True (#138536)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Author: andreuu-tsai

Doc/library/asyncio-subprocess.rst

@@ -311,8 +311,16 @@ their completion.
 
       A ``None`` value indicates that the process has not terminated yet.
 
-      A negative value ``-N`` indicates that the child was terminated
-      by signal ``N`` (POSIX only).
+      For processes created with :func:`~asyncio.create_subprocess_exec`, a negative
+      value ``-N`` indicates that the child was terminated by signal ``N``
+      (POSIX only).
+
+      For processes created with :func:`~asyncio.create_subprocess_shell`, the
+      return code reflects the exit status of the shell itself (e.g. ``/bin/sh``),
+      which may map signals to codes such as ``128+N``. See the
+      documentation of the shell (for example, the Bash manual's Exit Status)
+      for details.
+
 
 
 .. _asyncio-subprocess-threads:

Doc/library/subprocess.rst

@@ -964,6 +964,11 @@ Reassigning them to new values is unsupported:
    A negative value ``-N`` indicates that the child was terminated by signal
    ``N`` (POSIX only).
 
+   When ``shell=True``, the return code reflects the exit status of the shell
+   itself (e.g. ``/bin/sh``), which may map signals to codes such as
+   ``128+N``. See the documentation of the shell (for example, the Bash
+   manual's Exit Status) for details.
+
 
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