xfs_io: add missed inode command into man page

xfs_io: add missed inode command into man page

There's an "inode" command in xfs_io, it's used to query physical
information about an inode. But there's not any information about
it in xfs_io and other related man pages. So document this command
in the xfs_io man page now.

[sandeen: include some of djwong's edits]

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>