Convert and enhance the man page of btrfs-qgroup.
The original man page for btrfs-qgroup subcommand is almost useless for
new user(like me), so adds more information on it.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Qu Wenruo