Update Choice widget documentation to reflect current class heirarchy

[David Smith]

The documentation has a section called "Widgets inheriting from the Select widget", see ​docs. It then goes on to talk about choice widgets including both Select and RadioSelect inputs.

Since 9ac4dbd7b53d187ca54f28e247d3a120660938ca not all choice widgets inherit from the Select widget. Currently, both Select and RadioSelect are subclasses of the undocumented ChoiceWidget.

I've drafted a possible patch for discussion

Widgets with choices
====================

There are a group of widgets which deal with choices and present the user with
a list of options to choose from. The different widgets present this choice
differently; the :class:`Select` widget uses a ``<select>`` HTML  list
representation, while :class:`RadioSelect` uses radio buttons and
:class:`CheckboxSelectMultiple` uses checkboxes.