btrfs-progs: fix typedef

Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: Add a brief explanation of btrfs property in man 8 btrfs

As Chris reported at the following mail, although
btrfs property has its own manpage, man 8 btrfs-property,
there is no explanation about it in man 8 btrfs.

https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg40134.html

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reported-by: Chris Murphy <lists@colorremedies.com>
Signed-off-by: David Sterba <dsterba@suse.cz>