[css-properties-values] Add note about registering a property changing the computed value, which can affect transitions.

Author: tabatkins

css-properties-values-api/Overview.bs

@@ -192,6 +192,10 @@ a custom property is defined with a syntax of `<transform-function>#`,
 this will thus first interpolate as a simple list,
 and then each list item will interpolate as a 'transform' value.
 
+Note: Registering (or changing the registration) of a custom property
+can change its computed value,
+which can start or interrupt a CSS transition.
+
 Conditional Rules {#conditional-rules}
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