Improved venv docs to note that isolation is the default.

Author: facundobatista

Doc/library/venv.rst

@@ -22,8 +22,8 @@ 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,
+Python installation, known as the virtual environment's "base" Python, and by
+default is 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
 :ref:`virtual environments documentation <site-virtual-environments-configuration>`