bpo-23750: Document os-system, subprocess. Patch by Martin Panter. (GH-26016)

* Document os-system, subprocess Patch

* Update Doc/library/os.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>

Author: uniocto

Doc/library/os.rst

@@ -4211,12 +4211,12 @@ written in Python, such as a mail server's external command delivery program.
    the Standard C function :c:func:`system`, and has the same limitations.
    Changes to :data:`sys.stdin`, etc. are not reflected in the environment of
    the executed command. If *command* generates any output, it will be sent to
-   the interpreter standard output stream.
+   the interpreter standard output stream. The C standard does not
+   specify the meaning of the return value of the C function, so the return
+   value of the Python function is system-dependent.
 
    On Unix, the return value is the exit status of the process encoded in the
-   format specified for :func:`wait`.  Note that POSIX does not specify the
-   meaning of the return value of the C :c:func:`system` function, so the return
-   value of the Python function is system-dependent.
+   format specified for :func:`wait`.
 
    On Windows, the return value is that returned by the system shell after
    running *command*.  The shell is given by the Windows environment variable

Doc/library/subprocess.rst

@@ -1292,11 +1292,17 @@ Replacing :func:`os.system`
 
    sts = os.system("mycmd" + " myarg")
    # becomes
-   sts = call("mycmd" + " myarg", shell=True)
+   retcode = call("mycmd" + " myarg", shell=True)
 
 Notes:
 
 * Calling the program through the shell is usually not required.
+* The :func:`call` return value is encoded differently to that of
+  :func:`os.system`.
+
+* The :func:`os.system` function ignores SIGINT and SIGQUIT signals while
+  the command is running, but the caller must do this separately when
+  using the :mod:`subprocess` module.
 
 A more realistic example would look like this::