minor #10387 [Form] Deprecate searchAndRenderBlock returning empty string (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #10387).

Discussion
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[Form] Deprecate searchAndRenderBlock returning empty string

Fixes #10340.

Commits
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ac2ea07 [Form] Deprecate searchAndRenderBlock returning empty string

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -122,7 +122,7 @@ link for details), create a ``shipping_widget`` block to handle this:
         {% spaceless %}
             {% if expanded %}
                 <ul {{ block('widget_container_attributes') }}>
-                {% for child in form %}
+                {% for child in form if not child.rendered %}
                     <li>
                         {{ form_widget(child) }}
                         {{ form_label(child) }}
@@ -136,6 +136,12 @@ link for details), create a ``shipping_widget`` block to handle this:
         {% endspaceless %}
     {% endblock %}
 
+.. note::
+
+    Symfony 4.2 deprecated calling ``FormRenderer::searchAndRenderBlock`` for
+    fields that have already been rendered. That's why the previous example
+    includes the ``... if not child.rendered`` statement.
+
 .. tip::
 
     You can further customize the template used to render each children of the