Advise people to omit $FreeBSD$ unless the code is definitely going to
be merged to stable/12. This strengthens previous statements and is
appropriate now that stable/11 is no longer supported. If people are
wrong and things are unexpected merged to 12, tags can be added before
that merge. No sense adding a tag that will never be expanded and
removed later on the off chance it might wind up in stable/12.
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