Rose Edit: Properly document widgets

[wxtim]

The only reference I can find to the assorted widgets in the documentation are here. They do not reference all of the widgets.

Perhaps we could create documention like that for https://github.com/metomi/rose/blob/2c8956a9464bd277c8eb24d38af4803cba4c1243/lib/python/rose/config_editor/valuewidget/choice.py#L35 and use that with Sphinx autodocumentation?