panfrost/midgard: Document sign-extension/zero-extension bits (vector)

panfrost/midgard: Document sign-extension/zero-extension bits (vector)

For floating point ops, these bits determine the "negate?" and "abs?"
modifiers. For integer ops, it turns out they control how sign/zero
extension work, useful for mixing types.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>