docs($http): Adds detail about param sorting.

src/ng/http.js

@@ -689,7 +689,7 @@ function $HttpProvider() {
      *    - **url** – `{string}` – Absolute or relative URL of the resource that is being requested.
      *    - **params** – `{Object.<string|Object>}` – Map of strings or objects which will be turned
      *      to `?key1=value1&key2=value2` after the url. If the value is not a string, it will be
-     *      JSONified.
+     *      JSONified. The keys will be sorted when appended to the URL.
      *    - **data** – `{string|Object}` – Data to be sent as the request message data.
      *    - **headers** – `{Object}` – Map of strings or functions which return strings representing
      *      HTTP headers to send to the server. If the return value of a function is null, the