add form name caution to choice_name option

Author: xabbuh

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

@@ -9,3 +9,10 @@ of the choice views in the template.
 
 This can be a callable or a property path. See `choice_label`_ for similar usage.
 If ``null`` is used, an incrementing integer is used as the name.
+
+.. caution::
+
+    The configured value must be a valid form name. Make sure to only return
+    valid names when using a callable. Valid form names must be composed of
+    letters, digits, underscores, dashes and colons and must not start with
+    a dash or a colon.