* gnu/build/file-systems.scm (ext2-superblock-volume-name)
(bcachefs-superblock-volume-name, btrfs-superblock-volume-name)
(fat32-superblock-volume-name, fat16-superblock-volume-name)
(iso9660-superblock-volume-name, jfs-superblock-volume-name)
(f2fs-superblock-volume-name): Mention the file system type in the
docstring for consistency with the other superblock procedures.