docs($resource): add note about _promises as timeout_ not being supported

Fixes part of angular#13393.

Author: gkalpak

src/ngResource/resource.js

@@ -150,8 +150,10 @@ function shallowClearAndCopy(src, dst) {
  *     GET request, otherwise if a cache instance built with
  *     {@link ng.$cacheFactory $cacheFactory}, this cache will be used for
  *     caching.
- *   - **`timeout`** – `{number|Promise}` – timeout in milliseconds, or {@link ng.$q promise} that
- *     should abort the request when resolved.
+ *   - **`timeout`** – `{number}` – timeout in milliseconds.<br />
+ *     **Note:** In contrast to {@link ng.$http#usage $http.config}, {@link ng.$q promises} are
+ *     **not** supported in $resource, because the same value has to be re-used for multiple
+ *     requests.
  *   - **`withCredentials`** - `{boolean}` - whether to set the `withCredentials` flag on the
  *     XHR object. See
  *     [requests with credentials](https://developer.mozilla.org/en/http_access_control#section_5)