[css-animations-2] Define animation-trigger-* as reset-only sub-properties of animation (#11681)

Author: cdoublev

css-animations-2/Overview.bs

@@ -720,6 +720,9 @@ Declaring Animation Triggers</h2>
     and each item in the [=coordinated value list=]
     defining the properties of a single animation trigger.
 
+    The 'animation-trigger' properties
+    are [=reset-only sub-properties=] of the 'animation' [=shorthand=].
+
 ## The 'animation-trigger-type' property ## {#animation-trigger-type}
 
 The 'animation-trigger-type' property specifies the [=animation trigger type=]