VIRTIO-81: MUST vs must

Clean up the remaining lowercase "musts".  We actually introduce a
new normative section in the balloon; for the rest we clarify them
one way or another.

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
VIRTIO-74: required vs REQUIRED.

Some really are clearest as they are, but most are changed to avoid
confusion.

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
VIRTIO-73: MAY/may.

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