Add backfill details for Geo in the docs

doc/administration/geo/index.md

@@ -257,6 +257,16 @@ For more information on tuning Geo, see [Tuning Geo](replication/tuning.md).
 
 For an example of how to set up a location-aware Git remote URL with AWS Route53, see [Location-aware Git remote URL with AWS Route53](replication/location_aware_git_url.md).
 
+### Backfill
+
+Once a **secondary** node is set up, it will start replicating missing data from
+the **primary** node in a process known as **backfill**. You can monitor the
+synchronization process on each Geo node from the **primary** node's **Geo Nodes**
+dashboard in your browser.
+
+Failures that happen during a backfill are scheduled to be retried at the end
+of the backfill.
+
 ## Remove Geo node
 
 For more information on removing a Geo node, see [Removing **secondary** Geo nodes](replication/remove_geo_node.md).

doc/administration/geo/replication/troubleshooting.md

@@ -425,6 +425,11 @@ GitLab you are running. GitLab versions 11.11.x or 12.0.x are affected by
 
 To resolve the issue, upgrade to GitLab 12.1 or newer.
 
+### Failures during backfill
+
+During a [backfill](../index.md#backfill), failures are scheduled to be retried at the end
+of the backfill queue, therefore these failures only clear up **after** the backfill completes.
+
 ### Resetting Geo **secondary** node replication
 
 If you get a **secondary** node in a broken state and want to reset the replication state,