add missing qldoc (that was already missing?)

java/ql/lib/semmle/code/java/security/ExternalAPIs.qll

@@ -126,7 +126,9 @@ class UntrustedExternalApiDataNode extends ExternalApiDataNode {
 /** DEPRECATED: Alias for UntrustedExternalApiDataNode */
 deprecated class UntrustedExternalAPIDataNode = UntrustedExternalApiDataNode;
 
+/** An external API which is used with untrusted data. */
 private newtype TExternalApi =
+  /** An untrusted API method `m` where untrusted data is passed at `index`. */
   TExternalApiParameter(Method m, int index) {
     exists(UntrustedExternalApiDataNode n |
       m = n.getMethod() and