Enhance documentation for .add method middleware (#2246)

Clarified the types of middlewares that can be registered in the .add
method.

---------

Co-authored-by: René Jeglinsky <[email protected]>

Author: johannes-vogel

node.js/cds-serve.md

@@ -252,8 +252,11 @@ It adds the currently active model to the continuation. It's required for all ap
 ### .add(mw, pos?) {.method}
 
 Registers additional middlewares at the specified position.
-`mw` must be a function that returns an express middleware.
-`pos` specified the index or a relative position within the middleware chain. If not specified, the middleware is added to the end.
+`mw` can be either of:
+- a function that returns an express middleware
+- an express middleware with the common _req_, _res_, _next_ arguments
+- an array of express middlewares
+`pos` specifies the index or a relative position within the middleware chain. If not specified, the middleware is added to the end.
 
 ```js
 cds.middlewares.add (mw, {at:0}) // to the front