[css-scroll-snap] Clarify that Scroll Snap doesn't require any particular input method. Fixes #1305.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -837,6 +837,9 @@ Types of Scrolling Methods {#scroll-types}
     An <a>axis-locked</a> scroll is bound to only scroll along that axis.
     This prevents less-precise input mechanisms from drifting in the non-primary axis.
 
+    Note: This specification only applies to scrolling methods supported by the user agent.
+    It does not require the user agent to support any particular input or scrolling method.
+
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