Add usage examples for X.H.IndependentScreens.onCurrentScreen

[geekosaur]

Problem Description

onCurrentScreen is usable but you have to know all the internal types to know how to use it. It needs examples, or at bare minimum an indication that it wraps a StackSet operation that takes a PhysicalWorkspace and makes it take a VirtualWorkspace instead (for example, onCurrentScreen W.shift "2").

Checklist

• I've read CONTRIBUTING.md

• I tested my configuration

  • [n/a] With xmonad version XXX (commit XXX if using git)
  • [n/a] With xmonad-contrib version XXX (commit XXX if using git)