[css-values] Clarify that sqrt() and pow(X, .5) aren't *exactly* equivalent, in the case of -Infinity. JS is weird.

Author: tabatkins

css-values-4/Overview.bs

@@ -2760,7 +2760,8 @@ Exponential  Functions: ''pow()'', ''sqrt()'', ''hypot()''</h3>
 	which must resolve to a <<number>>,
 	and returns the square root of the value
 	as a <<number>>.
-	(''sqrt(X)'' and ''pow(X, .5)'' are equivalent;
+	(''sqrt(X)'' and ''pow(X, .5)'' are basically equivalent,
+	differing only in some error-handling;
 	''sqrt()'' is a common enough function
 	that it is provided as a convenience.)