Document that authGroup must have Access Database

Bug: Issue 13786
Change-Id: Iaf65252b25b9c40e5cfd1ac25d55fbf70536f83e

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 8d25b4e..06f5f48 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -374,6 +374,12 @@
 	By default, replicates without group control, i.e. replicates
 	everything to all remotes.
 
+	*NOTE:* If an authGroup is provided, and you want a complete
+	mirror (for backup reasons or to run a Gerrit replica), at
+	least one of the provided authGroups must have "Access Database"
+	capability. Otherwise [db](../../../Documentation/note-db.html)
+	refs will not be replicated.
+
 remote.NAME.createMissingRepositories
 :	If true, a repository is automatically created on the remote site.
 	If the remote site was not available at the moment when a new