[css-scrollbars] Editorial improvements to notes and advisements about scrollbar-width: none

See w3c#3315

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -206,19 +206,31 @@ which cannot be reasonably made thinner.
 In such cases, this value will behave as ''scrollbar-width/auto''.
 </dd>
 <dt><dfn>none</dfn></dt>
-<dd>implementations must not display any scrollbar, however the element's scrollability is not affected.
+<dd>Implementations must not display any scrollbar,
+however the element's scrollability by other means is not affected.
+
+<div class=advisement>
+Using this value can prevent mouse-only users from being able to scroll.
+Authors should ensure that mouse-only users can still reach hidden content,
+even if they have no scrollwheel.
+</div>
+
+<div class=advisement>
+Authors that use ''scrollbar-width/none'' should provide an alternative/equivalent visual hint that
+scrolling is possible and there is more content.
+</div>
+
+Note: For situations where an element is to be scrolled <em>only</em> by programmatic means,
+and not by direct user manipulation,
+authors should use ''overflow: hidden'' instead of ''scrollbar-width: none''.
+
 </dd>
 </dl>
 
 UAs must apply the 'scrollbar-width' value set on the root element to the viewport.
 
-Authors that use ''scrollbar-width/none'' should provide an alternative/equivalent visual hint that 
-scrolling is possible and there is more content. 
-For situations where an element is going to be scrolled by other means instead of direct user manipulation 
-(e.g. programmatically), authors should use ''overflow: hidden'' instead of ''scrollbar-width/none''.
-
-Note: Unlike 'overflow' (and overflow-*) properties, 
-a scrollbar-width value set on the HTML body element is not propagated to the viewport.
+Note: Unlike the 'overflow' property (and its longhands),
+a 'scrollbar-width' value set on the HTML <{body}> element is not propagated to the viewport.
 
 Note: This specification does not define the exact position or shape of the scrollbar, 
 or any animation thereof, such as fading or sliding in/out of view.