docs: clarify returncode behavior for subprocess_exec vs subprocess_shell

Update asyncio.subprocess returncode documentation:
- For create_subprocess_exec, negative values (-N) indicate termination by signal N (POSIX).
- For create_subprocess_shell, the return code follows the invoked shell’s exit status
  (e.g. 128+N for signals in bash/sh).

Author: andreuu-tsai

Doc/library/asyncio-subprocess.rst

@@ -305,8 +305,16 @@ 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:`create_subprocess_exec`, a negative
+      value ``-N`` indicates that the child was terminated by signal ``N``
+      (POSIX only).
+
+      For processes created with :func:`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: