Convert "Animatable" to "Animation type" prop definition field

Author: cdoublev

compositing-2/Overview.bs

@@ -125,7 +125,7 @@ Inherited: no
 Percentages: N/A
 Computed value: as specified
 Media: visual
-Animatable: no
+Animation type: discrete
 </pre>
 
 The syntax of the property of <<blend-mode>> is given with:
@@ -274,7 +274,7 @@ Inherited: no
 Percentages: N/A
 Computed value: as specified
 Media: visual
-Animatable: no
+Animation type: discrete
 </pre>
 
 The syntax of the property of <<isolation-mode>> is given with:
@@ -303,7 +303,7 @@ Inherited: no
 Percentages: N/A
 Computed value: as specified
 Media: visual
-Animatable: no
+Animation type: discrete
 </pre>
 
 The 'background-blend-mode' list must be applied in the same order as 'background-image' [[!CSS3BG]]. This means that the first element in the list will apply to the layer that is on top. If a property doesn't have enough comma-separated values to match the number of layers, the UA must calculate its used value by repeating the list of values until there are enough.</p>