Document command to upgrade vendored libraries

Author: pfmoore

src/pip/_vendor/README.rst

@@ -115,6 +115,16 @@ Vendoring is automated via the `vendoring <https://pypi.org/project/vendoring/>`
 Launch it via ``vendoring sync . -v`` (requires ``vendoring>=0.2.2``).
 Tool configuration is done via ``pyproject.toml``.
 
+To update the vendored library versions, we have a session defined in ``nox``.
+The command to upgrade everything is::
+
+    nox -s vendoring -- --upgrade-all --skip urllib3 --skip setuptools
+
+At the time of writing (April 2025) we do not upgrade ``urllib3`` because the
+next version is a major upgrade and will be handled as an independent PR. We also
+do not upgrade ``setuptools``, because we only rely on ``pkg_resources``, and
+tracking every ``setuptools`` change is unnecessary for our needs.
+
 
 Managing Local Patches
 ======================