Add doc for new parameter

Author: rowleya

SpiNNaker-allocserv/src/main/java/uk/ac/manchester/spinnaker/alloc/admin/UserControl.java

@@ -353,6 +353,9 @@ private static UserRecord sketchUser(Row row) {
 	 *
 	 * @param user
 	 *            The description of the user to create.
+	 * @param urlGen
+	 *            How to construct the URL for a group membership in the
+	 *            response. If {@code null}, the memberships will be omitted.
 	 * @return A description of the created user, or {@link Optional#empty()} if
 	 *         the user exists already.
 	 */