[css-text-3] Note about U+3000 not being affected by keep-all

Closes w3c#3142

Author: frivoal

css-text-3/Overview.bs

@@ -1000,6 +1000,10 @@ Line Breaking Details</h3>
         <p class=note>This is the other common behavior for Korean (which uses spaces between words),
         and is also useful for mixed-script text where CJK snippets are mixed
         into another language that uses spaces for separation.</dd>
+
+	Note: Despite its name, U+3000 IDEOGRAPHIC SPACE does not belong to the ID Unicode line breaking class
+	(it actually belongs to the BA class),
+	and the associated wrapping opportunity is <strong>not</strong> suppressed.
     </dl>
 
     <p>Symbols that line-break the same way as letters of a particular category