bpo-37403: Touch up venv docs (GH-14458)


Add a versionadded for PS Core and note that `.venv` is a common virtual environment name.
(cherry picked from commit f9f8e3ce709ceb15c8db8c8dde940daf1febf13d)

Co-authored-by: Brett Cannon <54418+brettcannon@users.noreply.github.com>
2 files changed