shield-dx and dy docs

[Tom MacWright]

mapbox/carto#359

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The docs state

shield-text-dy

Default Value: 0 Displace text within shield by fixed amount, in pixels, +/- along the Y axis. A positive value will shift the text down

shield-dy

Default Value: 0 Displace shield by fixed amount, in pixels, +/- along the Y axis. A positive value will shift the text down

Do both shift the text down, or does one shift the shield down?

I'd do a PR to fix it, but I'm not positive what all the alignment stuff does here.

[Saman Bemel-Benrud]

Looks like shield-dy moves both text and shield, text moves only text.