[css-text-3] Only consider allocated characters; unallocated characters shouldn't affect percentage threshold.

Author: fantasai

css-text-3/Overview.bs

@@ -5430,7 +5430,7 @@ Space-Discarding Unicode Characters</h2>
   ISSUE(5017): Should we classify punctuation / symbols as a category of space-ambiguous characters? (Currently spaces are discarded only if both sides are space-discarding; ambiguous characters would discard if the other side is space-discarding.)
 
   For future revisions of [[UNICODE]],
-  any new block whose contents comprise at least 50% codepoints belonging to the
+  any new block whose allocated contents comprise at least 50% codepoints belonging to the
   Han, Hiragana, Katakana, or Yi script
   shall also be considered part of the [=space-discarding character set=].