Correct method override information in WP_Widget class documentation

[Frank Klein]

The documentation at the top of the WP_Widget class file currently states:

This class must be extended for each widget and WP_Widget::widget(), WP_Widget::update() and WP_Widget::form() need to be overridden.

I'd say that this is probably not correct for WP_Widget::update() and WP_Widget::form():

1. WordPress does not enforce the overriding for these methods, as it does for WP_Widget::widget().
2. To have a widget without settings, not overriding these methods seems like the correct approach, as demonstrated by the contents of the WP_Widget::form() method.

I propose to rework the documentation to clarify this.

[DrewAPicture]

Hi @Frank-Klein, thanks for the patch!

[DrewAPicture]

In 37343:

Docs: Improve the class DocBlock for WP_Widget to clarify which methods "should" vs "must" be overridden by extending sub-classes.

Props Frank-Klein.
Fixes #36703.