btrfs-progs: send operates on ro snapshots only

btrfs-progs: send operates on ro snapshots only

While talking to another btrfs user on IRC today, it became clear that a
major point of confusion in the btrfs send manual is that it's not
telling the user soon enough that send/receive solely operates on
subvolume snapshots instead of the original (read/write) subvolumes.

So, change the first few lines to explicitly mention snapshots instead.
Technically, snapshots are also just subvolumes, but requiring this
level of technical detailed knowledge doesn't help the user who is just
trying out things.

Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Signed-off-by: David Sterba <dsterba@suse.com>