`HTMLLayerElement` documention for `extent` IDL attribute needed

[prushforth]

No .extent property documentation available. Should express the zoomTo() method in terms of that; make certain that it is consistent with HTMLFeatureElement.zoomTo(), HTMLFeatureElement.extent

[prushforth]

The extent IDL attribute is available on several map content elements, including HTMLLayerElement, HTMLExtentElement, and HTMLLinkElement. It should be mentioned in the docs for those elements.