The manuals are now evaluated with each derivation in `pkgs` (recursively)
replaced by a fake with path "\${pkgs.path.to.the.attribute}".
It isn't perfect, but it seems to cover a vast majority of use cases.
Caveat: even if the package is reached by a different means,
the path above will be shown and not e.g. `${config.services.foo.package}`.
As before, defaults created by `mkDefault` aren't displayed,
but documentation shouldn't (mostly) be a reason to use that anymore.
Note: t wouldn't be enough to just use `lib.mapAttrsRecursive`,
because derivations are also (special) attribute sets.
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