[#6297] Further clarifying this option

Author: weaverryan

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

@@ -6,22 +6,23 @@ choice_value
 
 **type**: ``callable`` or ``string`` **default**: ``null``
 
-Returns the string "value" for each choice. This is used in the ``value`` attribute
-in HTML and submitted in the POST/PUT requests. You don't normally need to worry
-about this, but it might be handy when processing an API request (since you can
-configure the value that will be sent in the API request).
+Returns the string "value" for each choice, which must be unique across all choices.
+This is used in the ``value`` attribute in HTML and submitted in the POST/PUT requests.
+You don't normally need to worry about this, but it might be handy when processing
+an API request (since you can configure the value that will be sent in the API request).
 
 This can be a callable or a property path. If ``null`` is given, an incrementing
 integer is used as the value.
 
-.. note::
+If you pass a callable, it will receive one argument: the choice itself. When using
+the :doc:`/references/forms/types/entity`, the argument will be the entity object
+for each choice or ``null`` in some cases, which you need to handle:
 
-    In contrast to other choice options (e.g. `choice_label`_), the callable for
-    the ``choice_value`` option takes as **only** argument the actual choice, which
-    is also the value that gets passed to your model when submitting the form.
+.. code-block:: php
 
-If you are using a callable to populate ``choice_value``, you need to check
-for the case that the value of the field may be ``null``.
+    'choice_value' => function (MyOptionEntity $entity = null) {
+        return $entity ? $entity->getId() : '';
+    },
 
 .. caution::