Add note about the promotion task being able to set Geo roles

In GitLab 13.5 or newer, it is safe to skip the step to
remove the Geo role manually from `etc/gitlab/gitlab.rb`
file since the proper roles will be enabled or disabled
during the promotion task.

doc/administration/geo/disaster_recovery/index.md

@@ -120,6 +120,10 @@ Note the following when promoting a secondary:
 1. Edit `/etc/gitlab/gitlab.rb` to reflect its new status as **primary** by
    removing any lines that enabled the `geo_secondary_role`:
 
+   Users of GitLab 13.5 or later can skip this step, due to the appropriate
+   roles being enabled or disabled during the promotion in the following
+   step.
+
    ```ruby
    ## In pre-11.5 documentation, the role was enabled as follows. Remove this line.
    geo_secondary_role['enable'] = true