[css-lists-3] Note text-space-collapse: preserve-spaces issue against ::marker

Author: fantasai

css-lists-3/Overview.bs

@@ -240,6 +240,9 @@ Properties Applying to ''::marker''</h4>
   needs to be explicitly specified in the [=selector=],
   e.g. ''::before::marker''.
 
+  ISSUE: ''white-space: pre'' doesn't have quite the right behavior;
+  ''text-space-collapse: preserve-spaces'' might be closer to what's needed here.
+
 <h3 id='content-property'>
 Generating Marker Contents</h3>