Document secondary promotion issue in pre-13.4

Add documentation on how to resolve the issue of promoting a
disabled secondary.

Relates to https://gitlab.com/gitlab-org/gitlab/issues/237896

doc/administration/geo/disaster_recovery/index.md

@@ -96,6 +96,10 @@ must disable the **primary** node.
 
 Note the following when promoting a secondary:
 
+- If replication was paused on the secondary node, for example as a part of upgrading,
+  while you were running a version of GitLab lower than 13.4, you _must_
+  [enable the node via the database](#while-promoting-the-secondary-i-got-an-error-activerecordrecordinvalid)
+  before proceeding.
 - A new **secondary** should not be added at this time. If you want to add a new
   **secondary**, do this after you have completed the entire process of promoting
   the **secondary** to the **primary**.
@@ -425,3 +429,20 @@ instructions in the
 [Upgrading to GitLab 10.5](../replication/version_specific_updates.md#updating-to-gitlab-105)
 section to resolve the error. Otherwise, the secret is lost and you'll need to
 [reset two-factor authentication for all users](../../../security/two_factor_authentication.md#disabling-2fa-for-everyone).
+
+### While Promoting the secondary, I got an error `ActiveRecord::RecordInvalid`
+
+If you disabled a secondary node, either with the [replication pause task](../replication/index.md#pausing-and-resuming-replication)
+(13.2) or via the UI (13.1 and earlier), you must first re-enable the
+node before you can continue. This is fixed in 13.4.
+
+From `gitlab-psql`, execute the following, replacing  `<your secondary url>`
+with the URL for your secondary server starting with `http` or `https` and ending with a `/`.
+
+```shell
+SECONDARY_URL="https://<secondary url>/"
+DATABASE_NAME="gitlabhq_production"
+sudo gitlab-psql -d "$DATABASE_NAME" -c "UPDATE geo_nodes SET enabled = true WHERE url = '$SECONDARY_URL';"
+```
+
+This should update 1 row.