switch groff parameter -man to -mandoc

groff 1.23.0 changed the semantics of the -man parameter, and many
manual pages are not rendered. The -mandoc parameter brings back
the old behavior, as in groff 1.22.4 and earlier.

PR: 273565, 273245

Reviewed by: emaste, bapt
MFC after: 1 week for all supported branches (stable/12, 13, 14)
Differential Revision: https://reviews.freebsd.org/D41737

(cherry picked from commit 035f7c9a3653d91564a5513f1311aa3b6e14a17e)