Documentation/ABI: Update IFS ABI doc

Array BIST test doesn't need an IFS test image to operate unlike
the SCAN test. Consequently current_batch and image_version
files are not applicable for Array BIST IFS device instance,
clarify this in the ABI doc.

Also given that multiple tests are supported, take the opportunity
to generalize descriptions wherever applicable.

Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20230322003359.213046-10-jithu.joseph@intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>