btrfs-progs: clarify mkfs.btrfs --alloc-start option

btrfs-progs: clarify mkfs.btrfs --alloc-start option

Fedora had a bug where a poor user thought that --alloc-start
meant that the filesystem would be created at that offset into
the device, rather than just starting allocations at that offset.
A subtle difference, but worth clarifying, because the manpage
is misleading on this point.

The original commit log for this option says:

    Add mkfs.btrfs -A offset to control allocation start on devices

    This is a utility option for the resizer, it makes sure to allocate
    at offset bytes in the disk or higher.  It ensures the resizer will have
    something to move when testing it.

so allude to that intended use in the manpage.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[converted to asciidoc]
Signed-off-by: David Sterba <dsterba@suse.cz>