btrfs-progs: add supported attr flags to btrfs(5)

btrfs-progs: add supported attr flags to btrfs(5)

The chattr(1) manpage suffers from the same problems mount(1)
had: many options listed, not kept up to date for various
filesystems.

I've submitted a manpage update for chattr(1) which says to
refer to filesystem-specific manpages for supported attributes;
this patch updates btrfs(5) to list the attributes supported
by btrfs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[added some asciidoc markups, adjusted formatting]
Signed-off-by: David Sterba <dsterba@suse.cz>