docs($http): correct and clarify default transforms

- baddata error described incorrect http behavior, and workarounds
- httpProvider defaults were missing transformResponse / transformRequest
- http was not clear about JSON detection strategy

Closes #15897 
Closes #15906

Author: Narretz

docs/content/error/$http/baddata.ngdoc

@@ -3,12 +3,12 @@
 @fullName Bad JSON Data
 @description
 
-The default @{link ng.$http#default-transformations `transformResponse`} will try to parse the
-response as JSON if the `Content-Type` header is `application/json` or the response looks like a
+The default {@link ng.$http#default-transformations `transformResponse`} will try to parse the
+response as JSON if the `Content-Type` header is `application/json`, or the response looks like a
 valid JSON-stringified object or array.
 This error occurs when that data is not a valid JSON object.
 
-The error message should provide additional context such as the actual response.
-
-To resolve this error, make sure you pass valid JSON data to `transformResponse` or use an
-appropriate `Content-Type` header for non-JSON data.
+To resolve this error, make sure you pass valid JSON data to `transformResponse`. If the response
+data looks like JSON, but has a different `Content-Type` header, you must
+{@link ng.$http#overriding-the-default-transformations-per-request implement your own response
+transformer on a per request basis}, or {@link ng.$http#default-transformations modify the default `$http` responseTransform}.

src/ng/http.js

@@ -266,12 +266,6 @@ function $HttpProvider() {
    * {@link ng.$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'`.
-   *
-   * - **`defaults.xsrfHeaderName`** - {string} - Name of HTTP header to populate with the
-   * XSRF token. Defaults value is `'X-XSRF-TOKEN'`.
-   *
    * - **`defaults.headers`** - {Object} - Default headers for all $http requests.
    * Refer to {@link ng.$http#setting-http-headers $http} for documentation on
    * setting default headers.
@@ -280,15 +274,38 @@ function $HttpProvider() {
    *     - **`defaults.headers.put`**
    *     - **`defaults.headers.patch`**
    *
+   * - **`defaults.jsonpCallbackParam`** - `{string}` - the name of the query parameter that passes the name of the
+   * callback in a JSONP request. The value of this parameter will be replaced with the expression generated by the
+   * {@link $jsonpCallbacks} service. Defaults to `'callback'`.
    *
    * - **`defaults.paramSerializer`** - `{string|function(Object<string,string>):string}` - A function
    *  used to the prepare string representation of request parameters (specified as an object).
    *  If specified as string, it is interpreted as a function registered with the {@link auto.$injector $injector}.
    *  Defaults to {@link ng.$httpParamSerializer $httpParamSerializer}.
    *
-   * - **`defaults.jsonpCallbackParam`** - `{string}` - the name of the query parameter that passes the name of the
-   * callback in a JSONP request. The value of this parameter will be replaced with the expression generated by the
-   * {@link $jsonpCallbacks} service. Defaults to `'callback'`.
+   * - **`defaults.transformRequest`** -
+   * `{Array<function(data, headersGetter)>|function(data, headersGetter)}` -
+   * An array of functions (or a single function) which are applied to the request data.
+   * By default, this is an array with one request transformation function:
+   *
+   *   - If the `data` property of the request configuration object contains an object, serialize it
+   *     into JSON format.
+   *
+   * - **`defaults.transformResponse`** -
+   * `{Array<function(data, headersGetter, status)>|function(data, headersGetter, status)}` -
+   * An array of functions (or a single function) which are applied to the response data. By default,
+   * this is an array which applies one response transformation function that does two things:
+   *
+   *  - If XSRF prefix is detected, strip it
+   *    (see {@link ng.$http#security-considerations Security Considerations in the $http docs}).
+   *  - If the `Content-Type` is `application/json` or the response looks like JSON,
+   *    deserialize it using a JSON parser.
+   *
+   * - **`defaults.xsrfCookieName`** - {string} - Name of cookie containing the XSRF token.
+   * Defaults value is `'XSRF-TOKEN'`.
+   *
+   * - **`defaults.xsrfHeaderName`** - {string} - Name of HTTP header to populate with the
+   * XSRF token. Defaults value is `'X-XSRF-TOKEN'`.
    *
    **/
   var defaults = this.defaults = {
@@ -552,15 +569,18 @@ function $HttpProvider() {
      *
      * Angular provides the following default transformations:
      *
-     * Request transformations (`$httpProvider.defaults.transformRequest` and `$http.defaults.transformRequest`):
+     * Request transformations (`$httpProvider.defaults.transformRequest` and `$http.defaults.transformRequest`) is
+     * an array with one function that does the following:
      *
      * - If the `data` property of the request configuration object contains an object, serialize it
      *   into JSON format.
      *
-     * Response transformations (`$httpProvider.defaults.transformResponse` and `$http.defaults.transformResponse`):
+     * Response transformations (`$httpProvider.defaults.transformResponse` and `$http.defaults.transformResponse`) is
+     * an array with one function that does the following:
      *
      *  - If XSRF prefix is detected, strip it (see Security Considerations section below).
-     *  - If JSON response is detected, deserialize it using a JSON parser.
+     *  - If the `Content-Type` is `application/json` or the response looks like JSON,
+   *      deserialize it using a JSON parser.
      *
      *
      * ### Overriding the Default Transformations Per Request