docs: clarify versioned override parent behavior

[ded-furby]

Summary

• clarify that a versioned parent override key also applies that spec to the matched parent package
• document how to keep the parent package on its existing spec with "." while only overriding children
• add a focused example that addresses the confusion in [DOCS] package.json overrides parent keySpec overrides parent version #9121

Testing

• docs-only change
• attempted local validation, but the fresh clone does not have npm/cli dev dependencies bootstrapped (node ./scripts/resetdeps.js is required before broader checks can run)

[owlstronaut]

Nice clarification, and the core point checks out. I confirmed in the arborist code and with a real install that a versioned key like @npm/bar@2.0.0 really does pin the parent, not just gate the nested overrides. One thing to fix though: the $ reference needs the full scoped name, so ".": "$bar" actually errors with Unable to resolve reference $bar. It should be ".": "$@npm/bar". The same bug exists in the "@npm/foo": "$foo" example just below it, so it might be worth fixing both while you're here.

[owlstronaut]

see above