snapshot: Various doc tweaks
    
    Since I was copying this text to form checkpoint XML and API
    documentation, I might as well make improvements along the way. Most
    of these changes are based on reviews of the checkpoint docs.
    
    Among other things: grammar tweaks, point to a single source of
    documentation rather than repeating verbosity, reword things for
    easier legibility.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>