Clarify in documentation that commentlinks are enabled by default

Change-Id: I63f31e1e6dfe9ed63ec677498904de3d86f6caea
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 6046ca7..e07f9cb 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -908,6 +908,8 @@
 user-supplied and thus can be verified to be XSS-free, but are only
 enabled for a subset of projects.
 +
+By default, true.
++
 Note that the names and contents of disabled sections are visible even
 to anonymous users via the
 link:rest-api-projects.html#get-config[REST API].