[css-values] Make tan() asymptotes alternate pos/neg, so adding/subtracting whole turns gives the same value. w3c#4101.

Author: tabatkins

css-values-4/Overview.bs

@@ -2634,8 +2634,10 @@ Argument Ranges</h4>
 
 	In ''tan(A)'', if A is one of the asymptote values
 	(such as ''90deg'', ''270deg'', etc),
-	the result must be +∞ for positive angles
-	and -∞ for negative angles.
+	the result must be +∞ for ''90deg'' and all values a multiple of ''360deg'' from that
+	(such as ''-270deg'' or ''450deg''),
+	and -∞ for ''-90deg'' and all values a multiple of ''360deg'' from that
+	(such as ''-450deg'' or ''270deg'').
 
 	Note: This is only relevant for units that can exactly represent the asymptotic values,
 	such as ''deg'' or ''grad''.