Document ioctl(2) operations of the agp(4) subsystem.  The original patch was obtained from [1], with heavy editing, and ammending text for some of the ioctls.
Document ioctl(2) operations of the agp(4) subsystem.  The original patch
was obtained from [1], with heavy editing, and ammending text for some of
the ioctls.

All ioctls (in sys/agpio.h) are now documented.

PR: docs/50503
Original patch by: Alex Semenyaka <alexs@snark.ratmir.ru> [1]

Content approved by:

	- Eric Anholt <anholt@FreeBSD.ORG>
	- Matthew N. Dodd <mdodd@FreeBSD.ORG> (cursory review)

Approved and Reviewed by: des (mentor), re (scottl)

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