docs(js): Add AxiosInstanceRequest docs

Author: GeekMasher

javascript/ql/lib/semmle/javascript/frameworks/ClientRequests.qll

@@ -289,6 +289,9 @@ module ClientRequest {
     }
   }
 
+  /**
+   * A model of a `axios` instance request.
+   */
   class AxiosInstanceRequest extends ClientRequest::Range, API::CallNode {
     string method;
     API::CallNode instance;