Update javadoc following ZK-4771

Steps to Reproduce

following ZK-4771, the ThemeProvider getThemeURIs method uris parameter may contain a stylesheet out of the box.

https://www.zkoss.org/javadoc/latest/zk/org/zkoss/zk/ui/util/ThemeProvider.html#getThemeURIs-org.zkoss.zk.ui.Execution-java.util.List-

contains stylesheet object [href: ~./zkmax/css/tablet.css.dsp type=text/css]

if zkmax is loaded

Current Result

documentation indicates that the method receives only String objects without customization

Expected Result

documentation should remove the entry regarding string only, or add more info regarding possible stylesheet in zkmax

Debug Information

also in classes implementing ThemeProvider

StandardThemeProvider, StandardThemeProvider, StandardThemeProvider, StandardThemeProvider, TabletThemeProvider

Workaround

 check for object type if objects are cast during custom ThemeProvider implementation