minor symfony#11018 Improved the explanation of the block_name form option (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Improved the explanation of the block_name form option

Fixes symfony#4330.

Commits
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baf24e0 Improved the explanation of the block_name form option

Author: javiereguiluz

reference/forms/types/options/block_name.rst.inc

@@ -4,6 +4,11 @@ block_name
 **type**: ``string`` **default**: the form's name (see :ref:`Knowing which
 block to customize <form-customization-sidebar>`)
 
-Allows you to override the block name used to render the form type.
-Useful for example if you have multiple instances of the same form and you
-need to personalize the rendering of the forms individually.
+Allows you to add a custom block name to the ones used by default to render the
+form type. Useful for example if you have multiple instances of the same form
+and you need to personalize the rendering of the forms individually.
+
+If you set for example this option to ``my_custom_name`` and the field is of
+type ``text``, Symfony will use the following names (and in this order) to find
+the block used to render the widget of the field: ``_my_custom_name_widget``,
+``text_widget`` and ``form_widget``.