[css-scroll-snap] Handle when snapping would cause overscroll.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -761,6 +761,11 @@ Choosing Snap Positions {#choosing}
 		and the final snapped scroll position,
 		subject to the additional constraints listed in this section.
 
+	* If the most appropriate <a>snap position</a> is "unreachable",
+		such that aligning to it would "overscroll" the  <a>scroll container</a>,
+		the <a>scroll container</a> must be scrolled <em>as much as possible</em> in each axis
+		toward the desired <a>snap position</a>.
+
 	* <a>2D snap positions</a> are all-or-nothing;
 		if a <a>2D snap position</a> is chosen to align to,
 		the <a>scroll container</a> must set its scroll position