bpo-40932: Note security caveat of shlex.quote on Windows (GH-21502)

Added a note in the `subprocess` docs that recommend using `shlex.quote` without mentioning that this is only applicable to Unix. 

Also added a warning straight into the `shlex` docs since it only says "for simple syntaxes resembling that of the Unix shell" and says using `quote` plugs the security hole without mentioning this important caveat.

Doc/library/shlex.rst

@@ -61,6 +61,20 @@ The :mod:`shlex` module defines the following functions:
    string that can safely be used as one token in a shell command line, for
    cases where you cannot use a list.
 
+   .. _shlex-quote-warning:
+
+   .. warning::
+
+      The ``shlex`` module is **only designed for Unix shells**.
+
+      The :func:`quote` function is not guaranteed to be correct on non-POSIX
+      compliant shells or shells from other operating systems such as Windows.
+      Executing commands quoted by this module on such shells can open up the
+      possibility of a command injection vulnerability.
+
+      Consider using functions that pass command arguments with lists such as
+      :func:`subprocess.run` with ``shell=False``.
+
    This idiom would be unsafe:
 
       >>> filename = 'somefile; rm -rf ~'

Doc/library/subprocess.rst

@@ -718,11 +718,8 @@ If the shell is invoked explicitly, via ``shell=True``, it is the application's
 responsibility to ensure that all whitespace and metacharacters are
 quoted appropriately to avoid
 `shell injection <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>`_
-vulnerabilities.
-
-When using ``shell=True``, the :func:`shlex.quote` function can be
-used to properly escape whitespace and shell metacharacters in strings
-that are going to be used to construct shell commands.
+vulnerabilities. On :ref:`some platforms <shlex-quote-warning>`, it is possible
+to use :func:`shlex.quote` for this escaping.
 
 
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