btrfs-progs: better document btrfs receive security

btrfs-progs: better document btrfs receive security

This adds some extra documentation to the btrfs-receive manpage that
explains some of the security related aspects of btrfs-receive.  The
first part covers the fact that the subvolume being received is writable
until the receive finishes, and the second covers the current lack of
sanity checking of the send stream.

Signed-off-by: Austin S. Hemmelgarn <ahferroin7@gmail.com>
Suggested-by: Graham Cobb <g.btrfs@cobb.uk.net>
Signed-off-by: David Sterba <dsterba@suse.com>