docs($http): link to usage where config is mentioned; make drier

Linking to usage section makes it easier for beginners to find out what the config object looks like.
The General Usage section now features an example that actually uses $http(config), and the Shortcut Methods section has been moved so that it appears directly after.

Closes #12949
Closes #12950

src/ng/http.js

@@ -425,28 +425,18 @@ function $HttpProvider() {
      *
      *
      * ## General usage
-     * The `$http` service is a function which takes a single argument — a configuration object —
+     * The `$http` service is a function which takes a single argument — a {@link $http#usage configuration object} —
      * that is used to generate an HTTP request and returns  a {@link ng.$q promise}.
      *
      * ```js
-     *   // Simple GET request example :
-     *   $http.get('/someUrl').
-     *     then(function(response) {
+     *   // Simple GET request example:
+     *   $http({
+     *     method: 'GET',
+     *     url: '/someUrl'
+     *   }).then(function successCallback(response) {
      *       // this callback will be called asynchronously
      *       // when the response is available
-     *     }, function(response) {
-     *       // called asynchronously if an error occurs
-     *       // or server returns response with an error status.
-     *     });
-     * ```
-     *
-     * ```js
-     *   // Simple POST request example (passing data) :
-     *   $http.post('/someUrl', {msg:'hello word!'}).
-     *     then(function(response) {
-     *       // this callback will be called asynchronously
-     *       // when the response is available
-     *     }, function(response) {
+     *     }, function errorCallback(response) {
      *       // called asynchronously if an error occurs
      *       // or server returns response with an error status.
      *     });
@@ -466,25 +456,16 @@ function $HttpProvider() {
      * XMLHttpRequest will transparently follow it, meaning that the error callback will not be
      * called for such responses.
      *
-     * ## Writing Unit Tests that use $http
-     * When unit testing (using {@link ngMock ngMock}), it is necessary to call
-     * {@link ngMock.$httpBackend#flush $httpBackend.flush()} to flush each pending
-     * request using trained responses.
-     *
-     * ```
-     * $httpBackend.expectGET(...);
-     * $http.get(...);
-     * $httpBackend.flush();
-     * ```
      *
      * ## Shortcut methods
      *
      * Shortcut methods are also available. All shortcut methods require passing in the URL, and
-     * request data must be passed in for POST/PUT requests.
+     * request data must be passed in for POST/PUT requests. An optional config can be passed as the
+     * last argument.
      *
      * ```js
-     *   $http.get('/someUrl').then(successCallback);
-     *   $http.post('/someUrl', data).then(successCallback);
+     *   $http.get('/someUrl', config).then(successCallback, errorCallback);
+     *   $http.post('/someUrl', data, config).then(successCallback, errorCallback);
      * ```
      *
      * Complete list of shortcut methods:
@@ -498,6 +479,17 @@ function $HttpProvider() {
      * - {@link ng.$http#patch $http.patch}
      *
      *
+     * ## Writing Unit Tests that use $http
+     * When unit testing (using {@link ngMock ngMock}), it is necessary to call
+     * {@link ngMock.$httpBackend#flush $httpBackend.flush()} to flush each pending
+     * request using trained responses.
+     *
+     * ```
+     * $httpBackend.expectGET(...);
+     * $http.get(...);
+     * $httpBackend.flush();
+     * ```
+     *
      * ## Deprecation Notice
      * <div class="alert alert-danger">
      *   The `$http` legacy promise methods `success` and `error` have been deprecated.
@@ -655,7 +647,7 @@ function $HttpProvider() {
      *
      * There are two kinds of interceptors (and two kinds of rejection interceptors):
      *
-     *   * `request`: interceptors get called with a http `config` object. The function is free to
+     *   * `request`: interceptors get called with a http {@link $http#usage config} object. The function is free to
      *     modify the `config` object or create a new one. The function needs to return the `config`
      *     object directly, or a promise containing the `config` or a new `config` object.
      *   * `requestError`: interceptor gets called when a previous interceptor threw an error or