[dom] Add "inclusive siblings" term

tabatkins has just created a new issue for 
https://github.com/whatwg/dom:

== Add "inclusive siblings" term ==
Selectors defines `:nth-child()`/etc in terms of index among siblings,
 and it's intended that if the element doesn't have a parent its the 
only sibling (so `:first-child` and `:last-child` both match, etc).  I
 need a term for "an element + its siblings".

See https://github.com/whatwg/dom/issues/55