Nettle.ECC_Curve: Improved documentation about point_mul().

Added note that point_mul() validates that the coordinate
is valid for the curve.

src/post_modules/Nettle/hogweed.cmod

@@ -492,6 +492,9 @@ PIKECLASS ECC_Curve
    *!
    *! @returns
    *!   Returns the new point on the curve.
+   *!
+   *! @throws
+   *!   Throws an error if the point (@[x], @[y]) isn't on the curve.
    */
   PIKEFUN array(object(Gmp.mpz)) point_mul(object(Gmp.mpz)|int x,
 					   object(Gmp.mpz)|int y,