Document userNameCaseInsensitive defaults to true for new sites

Change-Id: I83a7e21f7e79969b1922940416e8c3a6268f3838
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 0d51d6a..00501ed 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -676,6 +676,8 @@
 note name would be identical and thus conflict. These duplicates thus
 have to be deleted manually by deleting the respective external ID.
 +
+For newly initialized sites this option defaults to true.
++
 Default is false.
 
 [[auth.enableRunAs]]auth.enableRunAs::