[css-variables-1] Clarify the transition behavior of custom properties in a note. w3c#7405

Author: tabatkins

css-variables-1/Overview.bs

@@ -268,7 +268,7 @@ Defining Custom Properties: the '--*' family of properties</h2>
 	variable-reference-visited.html
 	</wpt>
 
-	Notably, they can even be transitioned or animated,
+	Notably, they can even be animated,
 	but since the UA has no way to interpret their contents,
 	they always use the "flips at 50%" behavior
 	that is used for any other pair of values that can't be intelligently interpolated.
@@ -288,6 +288,11 @@ Defining Custom Properties: the '--*' family of properties</h2>
 	variable-animation-to-only.html
 	</wpt>
 
+	Note: Like any other property that animates discretely,
+	custom properties can't be transitioned.
+	[=Registered custom properties=] can, however,
+	if given a syntax that has non-discrete animation behavior.
+
 	[=Animation-tainted=] is "infectious":
 	custom properties which reference [=animation-tainted=] properties
 	also become [=animation-tainted=].