bpo-40211: Clarify os.preadv and os.pwritev are supported on AIX 7.1+ (GH-19401)

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 943e676..6d5fb31 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1151,7 +1151,8 @@
    Combine the functionality of :func:`os.readv` and :func:`os.pread`.
 
    .. availability:: Linux 2.6.30 and newer, FreeBSD 6.0 and newer,
-      OpenBSD 2.7 and newer. Using flags requires Linux 4.6 or newer.
+      OpenBSD 2.7 and newer, AIX 7.1 and newer. Using flags requires
+      Linux 4.6 or newer.
 
    .. versionadded:: 3.7
 
@@ -1219,7 +1220,8 @@
    Combine the functionality of :func:`os.writev` and :func:`os.pwrite`.
 
    .. availability:: Linux 2.6.30 and newer, FreeBSD 6.0 and newer,
-      OpenBSD 2.7 and newer. Using flags requires Linux 4.7 or newer.
+      OpenBSD 2.7 and newer, AIX 7.1 and newer. Using flags requires
+      Linux 4.7 or newer.
 
    .. versionadded:: 3.7