service.8: Provide example for adding completions to (t)csh

service(8) has an example for bash completion, however bash is third
party and in /usr/share/examples/csh/dot.cshrc is a working example for
csh.

Since I use (t)csh, I've tested it, and it works for me.

PR: 179497
Submitted by: ohauer@

Reviewed by: kp (tentatively)
Differential Revision: https://reviews.freebsd.org/D28197