[css-scroll-snap] Clarify that 2-axis snapping does have *some* dependence between the two axises. Fixes #950.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -604,7 +604,13 @@ Scoping Valid Snap Positions to Visible Boxes
         we can default to a gridded behavior
         and introduce a switch to get smarter behavior later.)
     
-
+
+    Note: Although ''scroll-snap-type: both'' evaluates [=snap positions=] independently in each axis,
+    choosing of a [=snap position=] in one axis
+    may be influenced by [=snap positions=] in the other axis.
+    For example, snapping in one axis
+    may push off-screen the [=snap area=] that the other axis would otherwise align to,
+    making its [=snap position=] invalid and therefore unchooseable.
 
 
 Snapping Boxes that Overflow the Scrollport