[css-scrollbars] Simplify abstract

Broaden the scope to cover everything in the module.

Also avoids drawing attention to IE5.5: it did lead the way on this
topic, but that mention has made various people see this specification
in a negative light, assuming that what it's doing must be antiquated if
it's coming from something this old.

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -10,10 +10,9 @@ Repository: https://github.com/w3c/csswg-drafts/tree/master/css-scrollbars-1 css
 Editor: Tantek Çelik, Mozilla https://www.mozilla.org/, http://tantek.com/contact, w3cid 1464
 Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885
 Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net/, w3cid 43241
-Abstract: CSS Scrollbars standardizes the ability to style scrollbar controls, such as their color,
-          introduced in 2000 by Windows IE 5.5. 
-          This is useful when building web applications which use color schemes very different from 
-          the appearance of default platform scrollbars.
+Abstract: This CSS module defines properties
+          to influence the visual styling of scrollbars,
+          introducing controls for their color, width, and impact on layout.
 Link Defaults: css-color-3 (property) color
 </pre>