doc: enhance core dump instructions in admin guide

[garlick]

Problem: core dump note does not suggest systemd override.conf if not using coredumpctl(1).

Add an example.

[garlick]

restarting after bookworm failure here:

2024-05-01T15:30:07.1766443Z FAIL: t2212-job-manager-plugins.t 36 - job-manager: run all test plugin test modes
2024-05-01T15:30:07.1766521Z #	
2024-05-01T15:30:07.1766676Z #		cat <<-EOF | sort >test-modes.txt &&
2024-05-01T15:30:07.1766774Z #		priority unset
2024-05-01T15:30:07.1766864Z #		callback error
2024-05-01T15:30:07.1766964Z #		sched: priority unavail
2024-05-01T15:30:07.1767068Z #		sched: update priority
2024-05-01T15:30:07.1767196Z #		sched: dependency-add
2024-05-01T15:30:07.1767292Z #		sched: exception error
2024-05-01T15:30:07.1767390Z #		annotations error
2024-05-01T15:30:07.1767468Z #		EOF
2024-05-01T15:30:07.1767637Z #		COUNT=$(cat test-modes.txt | wc -l) &&
2024-05-01T15:30:07.1767733Z #		run_timeout 20 \
2024-05-01T15:30:07.1767886Z #		  flux bulksubmit --quiet --watch \
2024-05-01T15:30:07.1768064Z #		    --setattr=system.jobtap.test-mode={} \
2024-05-01T15:30:07.1768287Z #		    hostname :::: test-modes.txt >test-plugin.out &&
2024-05-01T15:30:07.1768443Z #		test_debug "cat test-plugin.out" &&
2024-05-01T15:30:07.1768635Z #		test $COUNT = $(cat test-plugin.out | wc -l) &&
2024-05-01T15:30:07.1768829Z #		flux jobs -ac $COUNT -no {annotations.test} | \
2024-05-01T15:30:07.1768979Z #		    sort >test-annotations.out &&
2024-05-01T15:30:07.1769169Z #		test_cmp test-modes.txt test-annotations.out
2024-05-01T15:30:07.1769246Z #	

[grondo]

LGTM!

[garlick]

Thanks!