[css-will-change] Add some more notes and clarifications to will-change:contents.

--HG--
extra : rebase_source : db2306cdce4d55b6f8d8e57be3b699a9b0eca04b

Author: tabatkins

css-will-change/Overview.bs

@@ -214,15 +214,27 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 		<dd>
 			Indicates that the author expects to animate or change something about the element's contents in the near future.
 
-			<p class='example'>
+			<div class='example'>
 				For example, browsers often “cache” rendering of elements over time,
 				because most things don't change very often,
 				or only change their position.
-				However, if an element <em>does</em> change its contents regularly,
+				However, if an element <em>does</em> change its contents continually,
 				producing and maintaining this cache is a waste of time.
 				A browser might take this value as a signal to cache less aggressively on the element,
 				or avoid caching at all and just continually re-render the element from scratch.
 
+				This value is mostly intended to help browsers optimize JS-based animations of content,
+				which change aspects of an element's contents many times per second.
+				This kind of optimization, when possible,
+				is already done automatically by browsers when declarative animations are used.
+			</div>
+
+			Note: This value more-or-less applies to the entire subtree of the element its declared on,
+			as it indicates the browser should count on *any* of the descendants changing in some way.
+			Using this on an element “high up” in your document might be very bad for your page's performance;
+			try to only use this on elements near the “bottom” of your document tree,
+			containing as little of the document as possible.
+
 		<dt><dfn><<custom-ident>></dfn>
 		<dd>
 			Indicates that the author expects to animate or change the property with the given name on the element in the near future.