Add documentation on how to fix a local patch during revendoring (#11918)

Author: pfmoore

src/pip/_vendor/README.rst

@@ -118,6 +118,30 @@ Vendoring is automated via the `vendoring <https://pypi.org/project/vendoring/>`
 ``pip/_vendor/vendor.txt`` and the different patches in
 ``tools/vendoring/patches``.
 Launch it via ``vendoring sync . -v`` (requires ``vendoring>=0.2.2``).
+Tool configuration is done via ``pyproject.toml``.
+
+
+Managing Local Patches
+======================
+
+The ``vendoring`` tool automatically applies our local patches, but updating,
+the patches sometimes no longer apply cleanly. In that case, the update will
+fail. To resolve this, take the following steps:
+
+1. Revert any incomplete changes in the revendoring branch, to ensure you have
+   a clean starting point.
+2. Run the revendoring of the library with a problem again: ``nox -s vendoring
+   -- --upgrade <library_name>``.
+3. This will fail again, but you will have the original source in your working
+   directory. Review the existing patch against the source, and modify the patch
+   to reflect the new version of the source. If you ``git add`` the changes the
+   vendoring made, you can modify the source to reflect the patch file and then
+   generate a new patch with ``git diff``.
+4. Now, revert everything *except* the patch file changes. Leave the modified
+   patch file unstaged but saved in the working tree.
+5. Re-run the vendoring. This time, it should pick up the changed patch file
+   and apply it cleanly. The patch file changes will be committed along with the
+   revendoring, so the new commit should be ready to test and publish as a PR.
 
 
 Debundling