Dialog: Extend buttons option docs to cover icons or other button widget options

[jzaefferer]

Currently the docs don't tell how to use icons with the buttons option. The relevant part of the implementation in dialog:

buttonOptions = {
    icons: props.icons,
    text: props.showText
};
delete props.icons;
delete props.showText;
$( "<button></button>", props )
    .button( buttonOptions )
    .appendTo( that.uiButtonSet );

In addition, the default for the buttons option is wrong, its an empty array.

[tjvantoll]

I fixed the default value of buttons.