Element: Add special case for document root in getClosestScrollableContainer
Element: Add special case for document root in getClosestScrollableContainer

Also documentation improvements, and correction for unattached elements.

Bug: T160852
Change-Id: Ifceac7e11ba0db2635f55d554f266fb13dc71807

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui