Update resolv.conf(5) "not necessary" language.

While there, fix mandoc -T lint warnings.

PR: 262492
Differential Revision: https://reviews.freebsd.org/D37449
MFC after: 1 week
Reviewed by: gjb
Approved by: gjb (mentor)

(cherry picked from commit e8261bba4e18ab95fe958e195b07a66df1b8d855)

Clarify what's normally not required.

Reported by: karels
Reviewed by: carlavilla, karels (informally)
Approved by: carlavilla (mentor), gjb (mentor, implicitly)
Differential Revision: https://reviews.freebsd.org/D37483

(cherry picked from commit 06703946d0be0baaf0f2a88f85e2dd5354e861da)