mdadm: add man page for --add-journal

mdadm: add man page for --add-journal

Add the following to man page:

--add-journal
      Recreate journal for RAID-4/5/6 array that lost a journal device.
      In the current implementation, this command cannot add a journal
      to an array that had a failed journal. To avoid interrupting
      on-going write opertion --add-journal only works for array in
      Read-Only state.

Reported-by: Yi Zhang <yizhan@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>