Improved venv docs to indicate that isolation is the default. (#136698)

* Improved venv docs to note that isolation is the default.

* Insert "that" so that a sentence reads better.

* Improved wording.

---------

Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk>

Author: facundobatista

Doc/library/venv.rst

@@ -22,10 +22,10 @@ The :mod:`!venv` module supports creating lightweight "virtual environments",
 each with their own independent set of Python packages installed in
 their :mod:`site` directories.
 A virtual environment is created on top of an existing
-Python installation, known as the virtual environment's "base" Python, and may
-optionally be isolated from the packages in the base environment,
-so only those explicitly installed in the virtual environment are available.
-See :ref:`sys-path-init-virtual-environments` and :mod:`site`'s
+Python installation, known as the virtual environment's "base" Python, and by
+default is isolated from the packages in the base environment,
+so that only those explicitly installed in the virtual environment are
+available. See :ref:`sys-path-init-virtual-environments` and :mod:`site`'s
 :ref:`virtual environments documentation <site-virtual-environments-configuration>`
 for more information.