Repair docs for math.frexp (they were wrong).

Reported on c.l.py by Kirill Simonov.

Modules/mathmodule.c

@@ -171,7 +171,9 @@ math_frexp(self, args)
 static char math_frexp_doc [] =
 "frexp(x)\n\
 \n\
-Return the matissa and exponent for x. The mantissa is positive.";
+Return the matissa and exponent of x, as pair (m, e).\n\
+m is a float and e is an int, such that x = m * 2.**e.\n\
+If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.";
 
 
 static PyObject *