Update doc on layout
Update doc on layout

- explain that references are resolved dynamically
- include "prev()" references
- update code example

Change-Id: If3c6eac04afd714d5f87551f5e7e07bd46e80ecd

moham3d/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js,mkostikov/tabris-js,eclipsesource/tabris-js,pimaxdev/tabris,mkostikov/tabris-js,pimaxdev/tabris,moham3d/tabris-js