release(7): Remove stray references to DOC* variables

We now live in the world of git, and release(7) should reflect that.

As of the commit referenced below, release images also no longer
include (stale) documentation, as the documentation has moved to
AsciiDoctor. This means that a few environment variables no longer
make sense, so remove them from their sections and mention them in
the compatibility section instead.

While here, also pet mandoc.

PR: 253615
MFC after: 3 days
MFC with: f61e92ca5a23 release: permanently remove the 'reldoc'
target and associates

Reviewed by: gjb, lwhsu, yuripv
Differential Revision: https://reviews.freebsd.org/D28881