Merge branch 'mk/geo/update-broadcast-message-docs' into 'master'

Update broadcast message notes for Geo

See merge request gitlab-org/gitlab-ee!5602

doc/administration/geo/replication/index.md

@@ -206,7 +206,6 @@ extra limitations may be in place.
 - The installation takes multiple manual steps that together can take about an hour depending on circumstances; we are
   working on improving this experience, see [gitlab-org/omnibus-gitlab#2978] for details.
 - Real-time updates of issues/merge requests (e.g. via long polling) doesn't work on the secondary
-- Broadcast messages set on the primary won't be seen on the secondary without a cache flush (e.g. gitlab-rake cache:clear)
 - [Selective synchronization](configuration.md#selective-synchronization)
   applies only to files and repositories. Other datasets are replicated to the
   secondary in full, making it inappropriate for use as an access control

doc/administration/geo/replication/updating_the_geo_nodes.md

@@ -14,6 +14,15 @@ all you need to do is update GitLab itself:
    the tracking database is enabled.
 1. [Test](#check-status-after-updating) primary and secondary nodes, and check version in each.
 
+## Upgrading to GitLab 10.8
+
+Before 10.8, broadcast messages would not propagate without flushing the cache on the secondaries.
+This has been fixed in 10.8, but requires one last cache flush on each secondary node:
+
+```bash
+gitlab-rake cache:clear
+```
+
 ## Upgrading to GitLab 10.6
 
 In 10.4 we started to recommend that you define a password for database user (`gitlab`).