Fix: Legacy Widgets description of "callback widgets" is not shown.

[jorgefilipecosta]

Description

Currently, the callback widgets description is not being shown. Callback widgets can also have a description, by setting that property in the options array passed to wp_register_sidebar_widget.

How has this been tested?

I pasted the following code in functions.php:

function my_marquee_widget_callback() {
	echo '<marquee>This is a legacy widget!<marquee>';
}
wp_register_sidebar_widget(
	'my-marquee-widget',
	'My marquee widget',
	'my_marquee_widget_callback',
	array(
		'description' => 'my marquee widget callback'
	)
);

function my_marquee_widget_callback2() {
	echo '<marquee>This is a legacy widget2!<marquee>';
}
wp_register_sidebar_widget(
	'my-marquee-widget2',
	'My marquee widget2',
	'my_marquee_widget_callback2'
);

I selected "My marquee widget" in the legacy widget block. I checked a description appears for this block. In master, it does not appear.
I selected "My marquee widget2" in the legacy widget block. I checked a description does not appears for this block.