[doc] Document VIRTUAL_ENV environment variable (GH-21970)
diff --git a/Doc/using/venv-create.inc b/Doc/using/venv-create.inc
index c8f6e8f..8f850a7 100644
--- a/Doc/using/venv-create.inc
+++ b/Doc/using/venv-create.inc
@@ -126,6 +126,10 @@
| | PowerShell | PS C:\\> <venv>\\Scripts\\Activate.ps1 |
+-------------+-----------------+-----------------------------------------+
+When a virtual environment is active, the :envvar:`VIRTUAL_ENV` environment
+variable is set to the path of the virtual environment. This can be used to
+check if one is running inside a virtual environment.
+
You don't specifically *need* to activate an environment; activation just
prepends the virtual environment's binary directory to your path, so that
"python" invokes the virtual environment's Python interpreter and you can run