TCMU_MAILBOX_FLAG_CAP_OOOC was introduced, and userspace can check the flag
for out-of-order completion capability support.

Also update the document on how to use the feature.
Signed-off-by: NSheng Yang <sheng@yasker.org>
Reviewed-by: NAndy Grover <agrover@redhat.com>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>