[css-ui] Add note about excessive use of user-select:none

Closes w3c#1317

Author: frivoal

css-ui-4/Overview.bs

@@ -1033,6 +1033,25 @@ Omitted values are set to their initial values.
 		that would hamper usability
 		or violate common user expectations.
 
+		<div class=note>
+			Authors should be careful about not constraining the user unnecessarily.
+			For example, setting ''user-select: none'' on the root element,
+			and relaxing that restriction on a handful of elements the author judges useful to select can be frustrating to users:
+
+			* Clicking in empty areas to undo the current selection will no longer be effective
+			* The author may have overlooked some areas which should be selectable
+			* Areas may be added later to the page without remembering to make them selectable
+			* The user may want to select pieces of text other than the main content,
+			    for instance to look them up in a dictionary or in some translation tool,
+			    or to look for warnings and error messages in a search engine…
+
+			Instead, a good practice is for authors is to selectively apply ''user-select: none'' to elements
+			which seem likely to be accidentaly selected
+			when doing so would interfere with the more likely intended acction.
+			Accidentally leaving parts of the page that are unlikely to be intereacted with selectable
+			will likely cause much less frustration to users than the oposite.
+		</div>
+
 		<dt><dfn>contain</dfn>
 		<dd>UAs must not allow a selection which is started in this element
 		to be extended outside of this element.