func_door_button is a variant of func_button which can be attached to a door, so that the pair of them move together. This is basically for making lifts with attached buttons. While the door is in motion the attached button and any other buttons which target the door are disabled.
Set this key to the targetname of the door you wish to attach the button to. If you require several doors, each with their own attached buttons but all of them affected by by the same trigger, then give them each a unique targetname, and use targetname2 to give all the doors a single name to be triggered. In all respects besides this key, the func_door_button is identical to a regular button, and the usual keys of wait, delay, target apply.
func_door_button does not ignore non-door entities when seeking its targetname entity (even though it will only bind to a func_door). This is a coding error that may be fixed in subsequent versions. For compatibility, ensure that the targetname for the func_door_button to bind to is unique among all entities, not just door entities.