virtio-spec: small English/punctuation corrections

1. s/These are devices are/These devices are
2. s/Thefirst/The first
3. s/, Guest should/. Guest should

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor mod)
Make it clear that you can refuse VIRTIO_BALLOON_F_MUST_TELL_HOST.
This was not the original intention, but no implementation requires the
feature, and making it compulsory has unintended consequences (it is
unlike any other feature).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>