Add documentation for CommentValidator interface

Change-Id: I576f746852e7c4f25fc430c54a8d8db0b8e25ea9
diff --git a/Documentation/config-validation.txt b/Documentation/config-validation.txt
index 24932a8..cb953c1 100644
--- a/Documentation/config-validation.txt
+++ b/Documentation/config-validation.txt
@@ -122,6 +122,15 @@
 perform validation when an account is activated or deactivated via the Gerrit
 REST API or the Java extension API.
 
+[[review-comment-validation]]
+== Review comment validation
+
+
+The `CommentValidator` interface allows plugins to validate all review comments,
+i.e. inline comments, file comments and the review message. This works for the
+REST API, for `git push` when `--publish-comments` is used and for comments sent
+via email.
+
 
 GERRIT
 ------