bpo-33095: Add reference to isolated mode in -m and script option (GH-7764)

Attempt to make isolated mode easier to discover via additional inline documentation.
Co-Authored-By: Julien Palard <julien@palard.fr>

Doc/using/cmdline.rst

@@ -101,6 +101,11 @@ source.
    first element will be set to ``"-m"``). As with the :option:`-c` option,
    the current directory will be added to the start of :data:`sys.path`.
 
+   :option:`-I` option can  be used to run the script in isolated mode where
+   :data:`sys.path` contains neither the current directory nor the user's
+   site-packages directory. All :envvar:`PYTHON*` environment variables are
+   ignored, too.
+
    Many standard library modules contain code that is invoked on their execution
    as a script.  An example is the :mod:`timeit` module::
 
@@ -121,6 +126,7 @@ source.
    .. versionchanged:: 3.4
       namespace packages are also supported
 
+.. _cmdarg-dash:
 
 .. describe:: -
 
@@ -133,6 +139,8 @@ source.
 
    .. audit-event:: cpython.run_stdin "" ""
 
+.. _cmdarg-script:
+
 .. describe:: <script>
 
    Execute the Python code contained in *script*, which must be a filesystem
@@ -151,6 +159,11 @@ source.
    added to the start of :data:`sys.path` and the ``__main__.py`` file in
    that location is executed as the :mod:`__main__` module.
 
+   :option:`-I` option can  be used to run the script in isolated mode where
+   :data:`sys.path` contains neither the script's directory nor the user's
+   site-packages directory. All :envvar:`PYTHON*` environment variables are
+   ignored, too.
+
    .. audit-event:: cpython.run_file filename
 
    .. seealso::