We recently renamed `jj util mangen` to `jj install-manpages` without a
deprecation warning, as it was missed in the initial PR. Waleed then
fixed it in a follow-up, which led to a longer discussion in the Discord which revealed that
not everyone was aware of it and _what_ requires a warning based deprecation.
This initially categorizes the policy into user-visible UX changes and small
impact renames, like for a small usergroup like packagers and third-party dependencies.
If we have more hickups with users, this can be expanded.