addressed @javiereguiluz comment

Author: HeahDude

reference/forms/types/entity.rst

@@ -191,8 +191,9 @@ choice_name
 
 **type**: ``string``, ``callable`` or :class:`Symfony\\Component\\PropertyAccess\\PropertyPath` **default**: id
 
-By default the name of each field is the id of the entity, if it can be get in
-the class metadata by an internal id reader.
+By default the name of each field is the id of the entity, if it can be read
+from the class metadata by an internal id reader. Otherwise the process will
+fall back on an incremental integer.
 
 choice_value
 ~~~~~~~~~~~~
@@ -209,6 +210,11 @@ even with a custom ``query_builder`` to be loaded.
 If it can be usefull for API to use an entity property as string value, you
 will gain performances by letting this option set by default.
 
+.. note::
+
+    If the id cannot be read, for BC, the component checks if the class implements
+    ``__toString()`` and will use an incremental integer otherwise.
+
 choices
 ~~~~~~~