[css-text-4] Clarify that collapsing only happens within a line. (Fix #330.)

Author: fantasai

css-text-4/Overview.bs

@@ -1124,6 +1124,7 @@ Fullwidth Punctuation Collapsing</h4>
 	Unless 'text-spacing' is set to ''space-adjacent'' or ''text-spacing/none''
 	(or the font has proportional fullwidth punctuation glyphs),
 	the UA must collapse the space typically associated with such full width glyphs
+	when placed adjacently on a line
 	as follows:
 
 	<ul>