Apply doc changes from code review

Co-authored-by: Wes Todd <wes@wesleytodd.com>

README.md

@@ -116,8 +116,8 @@ Maps the specified path parameter `name` to a specialized param-capturing middle
 This function positions the middleware in the same stack as `.use`.
 
 If a `Promise` is returned from the `param_middleware` function, the router
-will attach an `onRejected` callback using `.then`. If the promise is rejected,
-`next` will be called with the rejected value, or an error if the value is falsy.
+will attach an `onRejected` callback using `.then(null, onRejected)`. If the promise is rejected,
+`next` will be called with the rejected value as the error, or `new Error...` if the value is falsy.
 
 Parameter mapping is used to provide pre-conditions to routes
 which use normalized placeholders. For example a _:user_id_ parameter