Fix docstring in helpers.template_entity (#86227)

homeassistant/helpers/template_entity.py

@@ -283,6 +283,8 @@ class TemplateEntity(Entity):
             Called to store the template result rather than storing it
             the supplied attribute. Passed the result of the validator, or None
             if the template or validator resulted in an error.
+        none_on_template_error
+            If True, the attribute will be set to None if the template errors.
 
         """
         assert self.hass is not None, "hass cannot be None"