[scroll-snap-2] Add scroll-start-target longhand details (#8822)

scroll-start-target is a shorthand. This change specifies its related
shorthands.

css-scroll-snap-2/Overview.bs

@@ -196,7 +196,7 @@ The 'scroll-start-target' property {#scroll-start-target}
 
  <pre class="propdef shorthand">
  Name: scroll-start-target
- Value: [ none | auto ]
+ Value: [ none | auto ]{1,2}
  </pre>
 
  This property is a shorthand property that sets all of the scroll-start-target-* longhands in one declaration.
@@ -334,3 +334,34 @@ Flow-relative Longhands for 'scroll-start' {#scroll-start-longhands-logical}
  </pre>
 
  ...
+Flow-relative Longhands for 'scroll-start-target' {#scroll-start-target-longhands-logical}
+--------------------------------------------------------------------------
+
+ <pre class="propdef">
+ Name: scroll-start-target-block, scroll-start-target-inline
+ Value: auto | none
+ Initial: none
+ Applies to: all elements
+ Inherited: no
+ Percentages: n/a
+ Computed Value: either of the keywords "none" or "auto"
+ Animation type: not animatable
+ </pre>
+
+ ...
+
+Physical Longhands for 'scroll-start' {#scroll-start-target-longhands-physical}
+----------------------------------------------------------------------
+
+ <pre class="propdef">
+ Name: scroll-start-target-x, scroll-start-target-y
+ Value: none | auto
+ Initial: none
+ Applies to: all elements
+ Inherited: no
+ Percentages: n/a
+ Computed value: either of the keywords "none" or "auto"
+ Animation type: not animatable
+ </pre>
+
+ ...