minor #10022 Document Twig test "rootform" (yceruto, javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
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Document Twig test "rootform"

Documents PR symfony/symfony#25236 and closes #8829, sorry for the delay.

Commits
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cf355c9 Reworded the code comments
423bfae Minor reword
740803e Reword
fd433a1 Document Twig test "rootform"

Author: javiereguiluz

reference/forms/twig_reference.rst

@@ -217,6 +217,28 @@ array).
 
     <option {% if choice is selectedchoice(value) %} selected="selected"{% endif %} ...>
 
+.. _form-twig-rootform:
+
+rootform
+~~~~~~~~
+
+This test will check if the current ``form`` does not have a parent form view.
+
+.. code-block:: twig
+
+    {# DON'T DO THIS: this simple check can't differentiate between a form having
+       a parent form view and a form defining a normal form field called 'parent' #}
+
+    {% if form.parent is null %}
+        {{ form_errors(form) }}
+    {% endif %}
+
+   {# DO THIS: this check is always reliable, even if the form defines a field called 'parent' #}
+
+    {% if form is rootform %}
+        {{ form_errors(form) }}
+    {% endif %}
+
 .. _`twig-reference-form-variables`:
 
 More about Form Variables

reference/twig_reference.rst

@@ -750,6 +750,20 @@ selectedchoice
 Checks if ``selectedValue`` was checked for the provided choice field. Using
 this test is the most effective way.
 
+rootform
+~~~~~~~~
+
+.. code-block:: twig
+
+    {% if form is rootform %}
+
+``form``
+    **type**: ``FormView``
+
+Checks if the given ``form`` does not have a parent form view. This is the only
+safe way of testing it because checking if the form contains a field called
+``parent`` is not reliable.
+
 Global Variables
 ----------------