[cssom-view-1][editorial] Add intro text to isVisible()

Author: tabatkins

cssom-view-1/Overview.bs

@@ -1108,6 +1108,14 @@ Note: The {{DOMRect}} object returned by {{Element/getBoundingClientRect()}} is
 </div>
 
 <div algorithm>
+    Note: The {{Element/isVisible()}} method
+    provides a set of simple checks
+    for whether an element is potentially "visible".
+    It defaults to a very simple and straightforward method
+    based on the [=box tree=],
+    but allows for several additional checks to be opted into,
+    depending on what precise notion of "visibility" is desired.
+
     The <dfn method for=Element>isVisible(|options|)</dfn> method
     must run these steps,
     when called on an element |this|: