improve inverse trigonometric help files

help/acot

@@ -14,6 +14,14 @@ DESCRIPTION
     Returns the inverse cotangent of x to a multiple of eps with error less in
     absolute value than .75 * eps.
 
+    This function is sometimes called arccot, is such that:
+
+	cot(acot(x)) = x
+
+    and such that:
+
+	acot(x) = atan(1/x)
+
 EXAMPLE
     ; print acot(2, 1e-5),  acot(2, 1e-10), acot(2, 1e-15), acot(2, 1e-20)
     0.46365 0.463647609 0.463647609000806 0.46364760900080611621