posix-users/input_and_output.rst: Update AIO docs

posix-users/input_and_output.rst: Update AIO docs

Updated documentation after some changes to the AIO interface. This commit adds updates after these MR:

cpukit/posix/aio*: Create notification at reque... (rtems/rtos/rtems!118 - merged)

cpukit/posix/aio*: added support for O_DSYNC (rtems/rtos/rtems!128 - merged)

cpukit/posix/aio*: Implemented lio_listio() (rtems/rtos/rtems!188 - merged)