[css-text-4] slightly more detail on text-wrap:balance

Author: astearns

css-text-4/Overview.bs

@@ -177,7 +177,11 @@ Text Wrap Settings: the 'text-wrap' property</h3>
 		<dt><dfn>balance</dfn>
 		<dd>
 			Same as ''text-wrap/normal'' for inline-level elements.
-			For block-level elements line breaks are chosen to minimize the deviation from the average line length over the entire element.
+			For block-level elements line breaks are chosen to minimize the deviation from the average line length over the entire element
+			(including lines that end in a forced break).
+			The exact algorithm is UA-defined.
+
+			UAs may treat this value as ''text-wrap/normal'' if there are more than ten lines to balance.
 
 	</dl>