docs/library/uos: urandom: Generalize description.

Don't give a guarantee of HW RNG, only a possibility of its usage.

docs/library/uos.rst

@@ -88,8 +88,8 @@ Functions
 
 .. function:: urandom(n)
 
-   Return a bytes object with n random bytes, generated by the hardware
-   random number generator.
+   Return a bytes object with n random bytes. Whenever possible, it is
+   generated by the hardware random number generator.
 
 .. only:: port_wipy