[css-text-4] Clarify behavior of omitted values. #5382

See #5382 (comment)

css-text-4/Overview.bs

@@ -1809,6 +1809,9 @@ White Space and Wrapping: the 'white-space' property</h2>
  Note: This shorthand combines both inheritable and non-inheritable properties.
  If this is a problem, please inform the CSSWG.
 
+ Unless otherwise specified, any omitted [=longhand=]
+ is set to its [=initial value=].
+
  The following table gives the normative mapping
  of the values of the [=shorthand=]’s special keywords
  to their equivalent [=longhand=] values.
@@ -1847,7 +1850,7 @@ White Space and Wrapping: the 'white-space' property</h2>
  </tbody>
  </table>
 
- Values have the following informative definitions:
+ These keywords have the following informative definitions:
 
  ISSUE: Remove these definitions once the tests annotations have been redistributed.