porters-handbook: reference tools for handling services

Mention that service name start and sysrc name_enable=YES should be
used in pkg messages, rather than invoking the rc script directly or
editing /etc/rc.conf.

Reviewed by: bcr (earlier), jrm, emaste
Event: Kitchener-Waterloo Hackathon 202305
Differential Revision: https://reviews.freebsd.org/D40217