[3.13] Improved venv docs to indicate that isolation is the default. (GH-136698)

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

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

* Improved wording.

---------
(cherry picked from commit 8e2f4b4)

Co-authored-by: Facundo Batista <facundo@taniquetil.com.ar>
Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk>

Author: facundobatista

Doc/library/venv.rst

@@ -22,9 +22,9 @@ 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.
+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.
 
 When used from within a virtual environment, common installation tools such as
 :pypi:`pip` will install Python packages into a virtual environment