doc: posix: async-io: clean up async-io documentation

The async-io documentation was in need of some beautification.

Rather than repeating the same thing inside of the table, just
mention that the whole option is expected to fail enosys.

At this time, there does not seem to be sufficient need for
implementing async io, so it is only present so that conforming
applications can still link properly.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

doc/services/portability/posix/option_groups/index.rst

@@ -627,18 +627,22 @@ Additional POSIX Options
 _POSIX_ASYNCHRONOUS_IO
 ++++++++++++++++++++++
 
+Functions part of the ``_POSIX_ASYNCHRONOUS_IO`` Option are not implemented in Zephyr but are
+provided so that conformant applications can still link. These functions will fail, setting
+``errno`` to ``ENOSYS``:ref:`†<posix_undefined_behaviour>`.
+
 .. csv-table:: _POSIX_ASYNCHRONOUS_IO
    :header: API, Supported
    :widths: 50,10
 
-    aio_cancel(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_error(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_fsync(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_read(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_return(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_suspend(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    aio_write(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
-    lio_listio(),yes (will fail with ``ENOSYS``:ref:`†<posix_undefined_behaviour>`)
+    aio_cancel(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_error(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_fsync(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_read(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_return(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_suspend(),yes :ref:`†<posix_undefined_behaviour>`
+    aio_write(),yes :ref:`†<posix_undefined_behaviour>`
+    lio_listio(),yes :ref:`†<posix_undefined_behaviour>`
 
 .. _posix_option_cputime: