[css-scroll-snap] Add missing auto value to scroll-padding longhands. w3c#3189

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -982,8 +982,8 @@ Physical Longhands for 'scroll-padding' {#padding-longhands-physical}
 
     <pre class="propdef">
     Name: scroll-padding-top, scroll-padding-right, scroll-padding-bottom, scroll-padding-left
-    Value: <<length-percentage>>
-    Initial: 0
+    Value: auto | <<length-percentage>>
+    Initial: auto
     Applies to: <a>scroll containers</a>
     Inherited: no
     Percentages: relative to the scroll container’s scrollport
@@ -1000,7 +1000,7 @@ Flow-relative Longhands for 'scroll-padding'  {#padding-longhands-logical}
 
     <pre class="propdef">
     Name: scroll-padding-inline-start, scroll-padding-block-start, scroll-padding-inline-end, scroll-padding-block-end
-    Value: <<length-percentage>>
+    Value: auto | <<length-percentage>>
     Initial: auto
     Applies to: <a>scroll containers</a>
     Inherited: no