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svn:author
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imp
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svn:date
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2020-03-31T22:41:53.051168Z
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V 239
Document universe better

Document the kernels and worlds targets. Document the TARGETS and EXTRA_TARGETS
variables.

Reviewed by: brooks, bdrewery, emaste (LGTM)
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D24212

END