gh-153711: Fix os.pipe2 availability directive syntax

The availability extension only accepts comma-separated known platforms
(optionally with 'not ' / '>= version'). Free-form prose after 'not WASI.'
was parsed as an unknown platform and failed Docs EPUB/doctest with
warnings-as-errors.

Author: Vamsi-klu

Doc/library/os.rst

@@ -1448,8 +1448,7 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
    Return a pair of file descriptors ``(r, w)`` usable for reading and writing,
    respectively.
 
-   .. availability:: Unix, not WASI.  Availability on macOS requires macOS 27.0
-      or later; availability on iOS requires iOS 27.0 or later.
+   .. availability:: Unix, not WASI, macOS >= 27.0, iOS >= 27.0.
 
    .. versionadded:: 3.3