feature symfony#7204 Added docs for JsonResponse::fromJsonString (javiereguiluz)

This PR was squashed before being merged into the 3.2 branch (closes symfony#7204).

Discussion
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Added docs for JsonResponse::fromJsonString

This fixes symfony#7174.

Commits
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e9a3be0 Added docs for JsonResponse::fromJsonString

Author: xabbuh

components/http_foundation.rst

@@ -548,13 +548,22 @@ class, which can make this even easier::
 
     use Symfony\Component\HttpFoundation\JsonResponse;
 
+    // if you know the data to send when creating the response
+    $response = new JsonResponse(array('data' => 123));
+
+    // if you don't know the data to send when creating the response
     $response = new JsonResponse();
-    $response->setData(array(
-        'data' => 123
-    ));
+    // ...
+    $response->setData(array('data' => 123));
+
+    // if the data to send is already encoded in JSON
+    $response = JsonResponse::fromJsonString('{ "data": 123 }');
+
+.. versionadded:: 3.2
+    The ``JsonResponse::fromJsonString()`` method was added in Symfony 3.2.
 
-This encodes your array of data to JSON and sets the ``Content-Type`` header
-to ``application/json``.
+The ``JsonResponse`` class sets the ``Content-Type`` header to
+``application/json`` and encodes your data to JSON when needed.
 
 .. caution::