Btrfs-progs: update usage message for cmds-restore

Mention that a target path argument is mandatory unless
the -l option is supplied. Also mention about the existence
of the -l option, which was previously not announced.

$ btrfs restore -v /dev/sdb3
usage: btrfs restore [options] <device>

    Try to restore files from a damaged filesystem (unmounted)

    -s              get snapshots
    -v              verbose
    -i              ignore errors
    -o              overwrite
    -t              tree location
    -f <offset>     filesystem location
    -u <block>      super mirror
    -d              find dir
$ echo $?
129

After specifying a target path, the command works as expected:

$ btrfs restore -v /dev/sdb3 files2/
Restoring files2/file1
Done searching
$ echo $?
0

V2: Updated command synopsis by suggestion of Anand Jain.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

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