feat(banned): add warning about performance impact of matching rules

changes/ce/feat-12499.en.md

@@ -3,3 +3,5 @@ Added ability to ban clients by extended rules:
 * by matching client's `username` to a regular expression;
 * by matching client's peer address to an CIDR range.
 
+Warning: large number of matching rules (not tied to a concrete clientid, username or host) will impact performance.
+