Correcting the mistake in 678e3c0d2d99

Merge from 3.3
Addresses Issue #18553: isatty is not Unix only.

Doc/library/os.rst

@@ -757,6 +757,8 @@ as internal buffering of data.
 
    As of Python 3.3, this is equivalent to ``os.pathconf(fd, name)``.
 
+   Availability: Unix.
+
 
 .. function:: fstat(fd)
 
@@ -801,8 +803,6 @@ as internal buffering of data.
    Return ``True`` if the file descriptor *fd* is open and connected to a
    tty(-like) device, else ``False``.
 
-   Availability: Unix.
-
 
 .. function:: lockf(fd, cmd, len)