Document that configured groups must be visible to uploader

Bug: Issue 12342
Change-Id: I27df1a43665af224a4b368bf9c279b2c53ee7961
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 1088535..d5f924e 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -50,7 +50,10 @@
 filter.\<filter\>.reviewer
 :	An account or a group name. Must be an exact match (case sensitive) with the
 	account's email address or username, or the group name.  Multiple `reviewer`
-	occurrences are allowed.
+	occurrences are allowed.\
+	**NOTE**: *Reviewers are added in the context of the uploader which means
+	that if a group is configured it needs to be visible to the uploader for
+	reviewers to be added.*
 
 ##Multiple filter matches