docs: Document how to fetch additional branches

Tools like reno depend on a complete git history. Simply unshallowing a
clone is not sufficient, as we specifically fetch from the branch or tag
that we are building docs for, e.g.:

   git fetch origin --force --prune --prune-tags --depth 50 refs/heads/main:refs/remotes/origin/main

Add documentation describing how you can correct this if necessary.

Signed-off-by: Stephen Finucane <stephen@that.guru>

docs/user/build-customization.rst

@@ -88,7 +88,8 @@ we recommend you use them as a starting point.
 Unshallow git clone
 ^^^^^^^^^^^^^^^^^^^
 
-Read the Docs does not perform a full clone on ``checkout`` job to reduce network data and speed up the build process.
+Read the Docs does not perform a full clone in the ``checkout`` job in order to reduce network data and speed up the build process.
+Instead, it performs a `shallow clone <https://git-scm.com/docs/shallow>`_ and only fetches the branch or tag that you are building documentation for.
 Because of this, extensions that depend on the full Git history will fail.
 To avoid this, it's possible to unshallow the :program:`git clone`:
 
@@ -104,6 +105,22 @@ To avoid this, it's possible to unshallow the :program:`git clone`:
        post_checkout:
          - git fetch --unshallow || true
 
+If your build also relies on the contents of other branches, it may also be necessary to re-configure git to fetch these:
+
+.. code-block:: yaml
+   :caption: .readthedocs.yaml
+
+   version: 2
+   build:
+     os: "ubuntu-20.04"
+     tools:
+       python: "3.10"
+     jobs:
+       post_checkout:
+         - git fetch --unshallow || true
+         - git config remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*' || true
+         - git fetch --all --tags || true
+
 
 Cancel build based on a condition
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^