VIRTIO-72: SHOULD/should

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
VIRTIO-63: Make sure examples are marked clearly.

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
VIRTIO-62: Explicit and specific.

Avoid these words where they are redundant.  This also lead me to
notice that we were not consistent in the use of the term
"device-specific configuration" in the PCI section, so cleaned that up
too.

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>