minor #16561 Add information to sub request (DamienDeSousa)

This PR was submitted for the 4.4 branch but it was squashed and merged into the 5.4 branch instead.

Discussion
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Add information to sub request

Adding some information about _format attribute default value because I spend some times to understand why this wasn't working for me. Now I know why it was not working, _format value was html by default and I get no error message. So I just share with orther developers this little update to avoid them spending the same time.

Thank you, have a nice day :)

Commits
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4421eaa Add information to sub request

Author: javiereguiluz

components/http_kernel.rst

@@ -711,6 +711,11 @@ look like this::
 
 .. _http-kernel-resource-locator:
 
+Also, note that by default, if the ``_format`` attribute is not set in your request, the value will be ``html``.
+So, if your sub request returns something else than ``html`` (like json for instance) you can set it by setting the ``_format`` attribute on the request::
+
+    $request->attributes->set('_format', 'json');
+
 Locating Resources
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