Systematize `builtins.fetchTree` docs #12770

Job	Run time
vm_tests	41s
Check Cachix and Docker secrets present for installer tests	1s
tests (ubuntu-latest)	31s
tests (macos-latest)	1m 2s
flake_regressions	0s
installer	0s
docker_push_image	0s
installer_test	0s
	2m 15s

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