nixos manuals: allow displaying package references

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.

lib/attrsets.nix

@@ -207,7 +207,7 @@ rec {
 
 
   /* Like `mapAttrsRecursive', but it takes an additional predicate
-     function that tells it whether to recursive into an attribute
+     function that tells it whether to recurse into an attribute
      set.  If it returns false, `mapAttrsRecursiveCond' does not
      recurse, but does apply the map function.  It is returns true, it
      does recurse, and does not apply the map function.