[css-ui-4] Add note on the interaction of user-select & editable content

Author: frivoal

css-ui-4/Overview.bs

@@ -511,6 +511,14 @@ Omitted values are set to their initial values.
 		If the element has descendants on which 'user-select' does not compute to ''user-select/none'',
 		these descendants must be included in a selection extending across the element.
 
+		Note: ''user-select: none'' on a non-editable descendant of an editable element
+		means that in addition to not being editable,
+		that descendant is also not deletable,
+		neither directly nor by attempting to include it in a broader selection
+		and then deleting that selection.
+		This matters for example in template-based editing,
+		where an editable template may contain sections which must be preserved.
+
 		As 'user-select' is a UI convenience mechanism,
 		not a copy protection mechanism,
 		the UA may provide an alternative way for the user