target/user: Report capability of handling out-of-order completions to userspace

target/user: Report capability of handling out-of-order completions to userspace

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: Sheng Yang <sheng@yasker.org>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Orabug: 25395066
(cherry picked from commit 32c76de3466ed2a875e36c140ac4e3800fdfab6e)
Signed-off-by: Kyle Fortin <kyle.fortin@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>