docs/library/machine: Remove conditional docs for rng function.

And instead list its availability explicitly.

docs/library/machine.rst

@@ -83,12 +83,6 @@ Power related functions
 Miscellaneous functions
 -----------------------
 
-.. only:: port_wipy
-
-    .. function:: rng()
-
-        Return a 24-bit software generated random number.
-
 .. function:: unique_id()
 
    Returns a byte string with a unique identifier of a board/SoC. It will vary
@@ -112,6 +106,12 @@ Miscellaneous functions
    above. The timeout is the same for both cases and given by *timeout_us* (which
    is in microseconds).
 
+.. function:: rng()
+
+   Return a 24-bit software generated random number.
+
+   Availability: WiPy.
+
 .. _machine_constants:
 
 Constants