[css-will-change] Specify that we use CI matching against property na…

…mes. Also add a note that unknown values are fine, since Chrome does this wrong. Fixes #3155.

css-will-change-1/Overview.bs

@@ -239,7 +239,8 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 
 		<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.
+			If the <<custom-ident>> is an [=ASCII case-insensitive=] match for the name of a built-in CSS property,
+			it indicates that the author expects to animate or change the property with the given name on the element in the near future.
 			If the property given is a shorthand,
 			it indicates the expectation for all the longhands the shorthand expands to.
 
@@ -260,6 +261,12 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 			which means that effects that happen through custom properties
 			do not count for the rules below that are conditioned on any non-initial value of a property causing something.
 
+			Note: Specifying a value that's not recognized as a property is fine;
+			it simply has no effect.
+			This allows you to safely specify <em>new</em> properties that exist in some user agents
+			without negatively affecting down-level user agents
+			that don't know about that property.
+
 			<p class='example'>
 				For example, browsers often handle elements with 'transform' set to a non-initial value very differently from normal elements,
 				perhaps rendering them to their own “GPU layer”