docs($http): note that default $http headers can be overridden

Closes angular#4750

Author: dmkc

src/ng/http.js

@@ -290,8 +290,9 @@ function $HttpProvider() {
      * with the lowercased HTTP method name as the key, e.g.
      * `$httpProvider.defaults.headers.get = { 'My-Header' : 'value' }.
      *
-     * Additionally, the defaults can be set at runtime via the `$http.defaults` object in the same
-     * fashion.
+     * The defaults can also be set at runtime via the `$http.defaults` object in the same
+     * fashion. In addition, you can supply a `headers` property in the config object passed when
+     * calling `$http(config)`, which overrides the defaults without changing them globally.
      *
      *
      * # Transforming Requests and Responses