blackhole.4: Describe better

Rewrite the document description to fit cleanly on one line and omit
crossreferences, to clean up apropos results.

MFC after: 3 days
Reviewed by: tuexen, cperciva
Differential Revision: https://reviews.freebsd.org/D52695

(cherry picked from commit 38f6a4467419f5b2344d184baa454c126e2919a9)