[css-values-4] Add explanatory sentence of what hypot() means, as suggested by Amelia.

Author: tabatkins

css-values-4/Overview.bs

@@ -2578,6 +2578,8 @@ Exponential  Functions: ''pow()'', ''sqrt()'', ''hypot()''</h3>
 	contains one or more comma-separated [=calculations=],
 	and returns the length of an N-dimensional vector
 	with components equal to each of the [=calculations=].
+	(That is,
+	the square root of the sum of the squares of its arguments.)
 	The argument [=calculations=] can resolve to any <<number>>, <<dimension>>, or <<percentage>>,
 	but must have the <em>same</em> [=determine the type of a calculation|type=],
 	or else the function is invalid;