Document map DOM elements

There are currently various references to the 'map viewport' and to overlaycontainers, but no description of what/where these are.
Should this be `ol-overlaycontainer` instead?  I think it might be clearer to say something about this being a CSS class name.  Maybe "if `false`, it is placed in an element that lets events pass through (with class `ol-overlaycontainer`)" or something.