Documentation on the HelpButton's default action is limited and difficult to find

It's easy, from the User Manual, to understand how to add a default button (https://github.com/enthought/traitsui/blob/master/docs/source/traitsui_user_manual/custom_view.rst#command-buttons-the-buttons-attribute) such as `HelpButton`, but there is no easy way, from there, to understand what defines their default action.

The HelpButton’s default action is the Handler’s `show_help()`, but its API doc doesn't specify that it can/should be overridden:

https://github.com/enthought/traitsui/blob/199d6f2a78153df0ad66b8caae5c9fb9d73dfff1/traitsui/handler.py#L161-L168

It actually refers to the [API Doc](https://docs.enthought.com/traitsui/api/traitsui.help.html) for [help.py](https://github.com/enthought/traitsui/blob/master/traitsui/help.py), which also doesn't help solving the issue ("how do I attach an action to a HelpButton?"), so this reference seems misleading.

The only place where this answer can be found is in the Handler user manual: https://github.com/enthought/traitsui/blob/master/docs/source/traitsui_user_manual/handler.rst#overriding-standard-methods but it's not necessarily the first place where one would look.