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2019-10-11T11:13:47.110957Z
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add superio.4 and superio.9  manual pages

This adds basic documentation on what the superio driver is and how
other drivers can interact with it.  I decided to also document
superio's ivar accessors.

Reviewed by:	bcr, brueffer (both manual contents only)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D21958

END