Group - 'iterate' documentation and guard

Update iterate documentation to cover usage of `args` and added a guard so
that the callback can be used without requiring that `args` is specified.

Ref. https://github.com/photonstorm/phaser/issues/1352
Group - additional documentation refinement

General tidying/corrections of Group documentation.
Documentation - Group

- Merged in group doc updates

- Added a DisplayObject stub type for better link-out across documentation