Mentioned the <optgroup> html tag explicitly

Author: ThomasLandauer

reference/forms/types/choice.rst

@@ -142,7 +142,8 @@ text that's shown to the user. But that can be completely customized via the
 Grouping Options
 ----------------
 
-You can easily "group" options in a select by passing a multi-dimensional choices array::
+You can easily group the ``<option>``s of a ``<select>`` into ``<optgroup>``s
+by passing a multi-dimensional ``choices`` array::
 
     use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
     // ...

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

@@ -6,9 +6,9 @@ group_by
 
 **type**: ``array``, ``callable`` or ``string`` **default**: ``null``
 
-You can easily "group" options in a select simply by passing a multi-dimensional
-array to ``choices``. See the :ref:`Grouping Options <form-choices-simple-grouping>`
-section about that.
+You can easily group the ``<option>``s of a ``<select>`` into ``<optgroup>``s,
+simply by passing a multi-dimensional array to ``choices``. See the
+:ref:`Grouping Options <form-choices-simple-grouping>` section about that.
 
 The ``group_by`` option is an alternative way to group choices, which gives you
 a bit more flexibility.
@@ -36,7 +36,7 @@ Take the following example::
     ));
 
 This groups the dates that are within 3 days into "Soon" and everything else into
-a "Later" group:
+a "Later" ``<optgroup>``:
 
 .. image:: /_images/reference/form/choice-example5.png
    :align: center