Add information to menu button pattern about usage constraints that mirrors usage information in combobox pattern

[carmacleod]

The combobox pattern contains the following prose:

Note that the listbox and menu button patterns are also useful in this scenario; differences between combobox and alternative patterns are described below.

and:

Two other widgets that are also visually compact and enable users to make a single choice from a set of discrete choices are listbox and menu button. One feature that distinguishes combobox from both listbox and menu button is that the user's choice can be presented as a value in an editable field, which gives users the ability to select some or all of the value for copying to the clipboard. Comboboxes and menu buttons can be implemented so users can explore the set of allowed choices without losing a previously made choice. ... ... a menu button cannot be marked required, and while it has an accessible name, it does not have a value so is not suitable for conveying the user's choice in its collapsed state.

Question: Do we really want authors to use the menu button pattern as a popup list, including dynamically changing the name of the button as the user makes menu item selections? I don't know - maybe it actually works in AT and is a valid thing to do? Just feels odd to me. Opening this issue to have the discussion.