fix(_internal/cli/cmdoptions): Add --require-virutalenv to docs

docs/html/user_guide.rst

@@ -428,6 +428,16 @@ set like this:
     ignore-installed = true
     no-dependencies = yes
 
+Similarly ``pip`` also provides an option ``require-virtualenv``, which can be
+set to true if an operation should be performed within a virtual environement.
+It would abort the operation if no active virtual environment is detected. You
+can set the option just as above:
+
+.. code-block:: ini
+
+    [install]
+    require-virtualenv = true
+
 To enable the boolean options ``--no-compile``, ``--no-warn-script-location``
 and ``--no-cache-dir``, falsy values have to be used:
 

news/7688.doc

@@ -0,0 +1 @@
+Document ``--require-virtualenv`` option, and how to use it.

src/pip/_internal/cli/cmdoptions.py

@@ -172,7 +172,10 @@ class PipOption(Option):
     dest='require_venv',
     action='store_true',
     default=False,
-    help=SUPPRESS_HELP
+    help=(
+        "Require a virtualenv to be active. Aborts when no active virtualenv "
+        "is detected."
+    ),
 )  # type: Callable[..., Option]
 
 verbose = partial(