docs($http): improve description of caching

Included changes:

* Point out that only GET & JSONP requests are cached.
* Explain that the URL+search params are used as cache keys (headers not considered).
* Add note about cache-control headers on response not affecting Angular caching.
* Mention `$httpProvider.defaults.cache` (in addition to `$http.defaults.cache`).
* Clear up how `defaults.cache` and `config.cache` are taken into account for determining the
  caching behavior for each request.

Fixes #11101
Closes #13003

Author: ryanhart2

src/ng/http.js

@@ -257,10 +257,9 @@ 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 `defaults.cache = false` then only requests that specify their own custom
-   * cache object will be cached. See {@link $http#caching $http Caching} for more information.
+   * - **`defaults.cache`** - {boolean|Object} - A boolean value or object created with
+   * {@link ng.$cacheFactory `$cacheFactory`} to enable or disable caching of HTTP responses
+   * by default. See {@link $http#caching $http Caching} for more information.
    *
    * - **`defaults.xsrfCookieName`** - {string} - Name of cookie containing the XSRF token.
    * Defaults value is `'XSRF-TOKEN'`.
@@ -608,26 +607,35 @@ function $HttpProvider() {
      *
      * ## Caching
      *
-     * To enable caching, set the request configuration `cache` property to `true` (to use default
-     * cache) or to a custom cache object (built with {@link ng.$cacheFactory `$cacheFactory`}).
-     * When the cache is enabled, `$http` stores the response from the server in the specified
-     * cache. The next time the same request is made, the response is served from the cache without
-     * sending a request to the server.
+     * {@link ng.$http `$http`} responses are not cached by default. To enable caching, you must
+     * set the config.cache value or the default cache value to TRUE or to a cache object (created
+     * with {@link ng.$cacheFactory `$cacheFactory`}). If defined, the value of config.cache takes
+     * precedence over the default cache value.
      *
-     * Note that even if the response is served from cache, delivery of the data is asynchronous in
-     * the same way that real requests are.
+     * In order to:
+     *   * cache all responses - set the default cache value to TRUE or to a cache object
+     *   * cache a specific response - set config.cache value to TRUE or to a cache object
      *
-     * If there are multiple GET requests for the same URL that should be cached using the same
-     * cache, but the cache is not populated yet, only one request to the server will be made and
-     * the remaining requests will be fulfilled using the response from the first request.
+     * If caching is enabled, but neither the default cache nor config.cache are set to a cache object,
+     * then the default `$cacheFactory($http)` object is used.
      *
-     * You can change the default cache to a new object (built with
-     * {@link ng.$cacheFactory `$cacheFactory`}) by updating the
-     * {@link ng.$http#defaults `$http.defaults.cache`} property. All requests who set
-     * their `cache` property to `true` will now use this cache object.
+     * The default cache value can be set by updating the
+     * {@link ng.$http#defaults `$http.defaults.cache`} property or the
+     * {@link $httpProvider#defaults `$httpProvider.defaults.cache`} property.
+     *
+     * When caching is enabled, {@link ng.$http `$http`} stores the response from the server using
+     * the relevant cache object. The next time the same request is made, the response is returned
+     * from the cache without sending a request to the server.
+     *
+     * Take note that:
+     *
+     *   * Only GET and JSONP requests are cached.
+     *   * The cache key is the request URL including search parameters; headers are not considered.
+     *   * Cached responses are returned asynchronously, in the same way as responses from the server.
+     *   * If multiple identical requests are made using the same cache, which is not yet populated,
+     *     one request will be made to the server and remaining requests will return the same response.
+     *   * A cache-control header on the response does not affect if or how responses are cached.
      *
-     * If you set the default cache to `false` then only requests that specify their own custom
-     * cache object will be cached.
      *
      * ## Interceptors
      *
@@ -805,10 +813,9 @@ function $HttpProvider() {
      *      by registering it as a {@link auto.$provide#service service}.
      *      The default serializer is the {@link $httpParamSerializer $httpParamSerializer};
      *      alternatively, you can use the {@link $httpParamSerializerJQLike $httpParamSerializerJQLike}
-     *    - **cache** – `{boolean|Cache}` – If true, a default $http cache will be used to cache the
-     *      GET request, otherwise if a cache instance built with
-     *      {@link ng.$cacheFactory $cacheFactory}, this cache will be used for
-     *      caching.
+     *    - **cache** – `{boolean|Object}` – A boolean value or object created with
+     *      {@link ng.$cacheFactory `$cacheFactory`} to enable or disable caching of the HTTP response.
+     *      See {@link $http#caching $http Caching} for more information.
      *    - **timeout** – `{number|Promise}` – timeout in milliseconds, or {@link ng.$q promise}
      *      that should abort the request when resolved.
      *    - **withCredentials** - `{boolean}` - whether to set the `withCredentials` flag on the