Improve documentation of `nix flake lock` #14009

Job	Run time
vm_tests	21m 34s
Check Docker secrets present for installer tests	1s
eval	54s
meson_build (ubuntu-latest)	39s
meson_build (macos-latest)	1m 33s
tests on ubuntu	22m 46s
tests on macos	14m 34s
flake_regressions	34m 48s
installer test on ubuntu	33s
installer test on macos	1m 24s
docker_push_image	0s
	1h 38m 46s

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