btrfs-progs: update manpage for btrfs-image with -m option added

The btrfs-image support multiple devices with -m specified.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/btrfs-image.txt

@@ -48,6 +48,9 @@ Walk all the trees manually and copy any blocks that are referenced. Use this
 option if your extent tree is corrupted to make sure that all of the metadata is
 captured.
 
+-m::
+Restore for multiple devices, more than 1 device should be provided.
+
 EXIT STATUS
 -----------
 *btrfs-image* will return 0 if no error happened.

btrfs-image.c

@@ -2446,6 +2446,7 @@ static void print_usage(void)
 	fprintf(stderr, "\t-o      \tdon't mess with the chunk tree when restoring\n");
 	fprintf(stderr, "\t-s      \tsanitize file names, use once to just use garbage, use twice if you want crc collisions\n");
 	fprintf(stderr, "\t-w      \twalk all trees instead of using extent tree, do this if your extent tree is broken\n");
+	fprintf(stderr, "\t-m	   \trestore for multiple devices\n");
 	exit(1);
 }