[selectors] Added a note about how at most one time-dimensional pseudo matches at a time.

Author: tabatkins

selectors/Overview.bs

@@ -2279,6 +2279,10 @@ Time-dimensional Pseudo-classes</h2>
 	If there is no timeline defined for an element,
 	these pseudo-classes must not match the element.
 
+	Note: Ancestors of a '':current'' element are also '':current'',
+	but ancestors of a '':past'' or '':future'' element are not necessarily '':past'' or '':future'' as well.
+	A given element matches at most one of '':current'', '':past'', or '':future''.
+
 <h3 id="the-current-pseudo">
 The Current-element Pseudo-class: '':current''</h3>