[css-will-change] Reword text according to dbaron's suggestions.

Author: tabatkins

css-will-change/Overview.bs

@@ -256,8 +256,9 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 			It is still <em>safe</em> to specify them, though;
 			it'll simply have no effect.
 
-			Specifying a <a>custom property</a> must have no effect,
-			including the "side-effect" management detailed below.
+			Specifying a custom property must have no effect,
+			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.
 
 			<p class='example'>
 				For example, browsers often handle elements with 'transform' set to a non-initial value very differently from normal elements,
@@ -277,7 +278,7 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 			If any non-initial value of a property would cause the element to generate a containing block for fixed positioned elements,
 			specifying that property in 'will-change' must cause the element to generate a containing block for fixed positioned elements.
 
-			If a non-initial value of a property would cause rendering differences on the element
+			If any non-initial value of a property would cause rendering differences on the element
 			(such as using a different anti-aliasing strategy for text),
 			the user agent should use that alternate rendering when the property is specified in 'will-change',
 			to avoid sudden rendering differences when the property is eventually changed.