Add the availability.

Author: junkmd

Doc/library/ctypes.rst

@@ -1809,8 +1809,7 @@ different ways, depending on the type and number of the parameters in the call:
 
 .. exception:: COMError(hresult, text, details)
 
-   Windows only: This non-public exception is raised when a COM method call
-   failed.
+   This non-public exception is raised when a COM method call failed.
 
    .. attribute:: hresult
 
@@ -1824,6 +1823,8 @@ different ways, depending on the type and number of the parameters in the call:
 
       The 5-tuple representing additional details about the error.
 
+   .. availability:: Windows
+
 
 The optional *paramflags* parameter creates foreign function wrappers with much
 more functionality than the features described above.