Clarify behavior of of overrides in CLI reference (#7537)
## Summary
Improve the description of override-dependencies based on the statement
in `concepts/resolution.md`: "As with constraints, overrides do not add
a dependency on the package and only take effect if the package is
requested in a direct or transitive dependency."
I tested it locally, `concepts/resolution.md` is correct. It would be
better to also include this in the Reference Chapter of the docs.
