docs($httpProvider): add info about defaults.cache

Closes angular#10134

Author: petebacondarwin

src/ng/http.js

@@ -116,6 +116,11 @@ function $HttpProvider() {
    *
    * Object containing default values for all {@link ng.$http $http} requests.
    *
+   * - **`defaults.cache`** - {Object} - an object built with {@link ng.$cacheFactory `$cacheFactory`}
+   * that will provide the cache for all requests who set their `cache` property to `true`.
+   * If you set the `default.cache = false` then only requests that specify their own custom
+   * cache object will be cached. See {@link $http#caching $http Caching} for more information.
+   *
    * - **`defaults.xsrfCookieName`** - {string} - Name of cookie containing the XSRF token.
    * Defaults value is `'XSRF-TOKEN'`.
    *
@@ -129,6 +134,7 @@ function $HttpProvider() {
    *     - **`defaults.headers.post`**
    *     - **`defaults.headers.put`**
    *     - **`defaults.headers.patch`**
+   *
    **/
   var defaults = this.defaults = {
     // transform incoming response data