Install and Update now carry multi-line /// doc comments documenting
per-pin resolution, target_dir semantics, and the lockfile's resolved
tag behaviour.  #[command(verbatim_doc_comment)] preserves the layout
under `--help` so the per-pin matrix renders correctly.

Internal helpers (run_install / run_add / run_update / update_dep /
run_clean / copy_dir_contents) now have rustdoc summarising their
contract for maintainers and contributors.