[css-ui-4] Include more details in issue about browser differences

Author: frivoal

css-ui-4/Overview.bs

@@ -185,6 +185,13 @@ Abstract: This is a delta specification over CSS-UI Level 3.
 		they behave as if the element has ''all'' and select it entirely.
 		IE does not restrict selections started outside of the element
 		from going into it at all.
+		Another difference is that in Chrome and Safari,
+		if the user attempts to start a selection inside a ''user-select: none'',
+		and to end the selection out of it,
+		a selection will be created from the boundary of the element
+		to the user-designated end-point.
+		Firefox and Internet explorer behave as prescribed in this specification
+		and do not create a selection at all.
 
 		However, if this element has descendants on which the computed value of 'user-select' is not ''user-select/none'',
 		selections that start and end within these descendants are allowed.