nix/docs: drop dead walk helper + dedupe checks.docs eval

argus review notes on #618:

- `walk` in `pickSubtrees` was leftover from an earlier traversal
  design; `pick` does everything we need. drop it.
- `checks.docs` was re-importing `nix/docs.nix` independently of
  `packages.docs`; the comment claimed they shared eval but they
  didn't (nix's lazy eval + import caching made the *result*
  identical, not the eval). switch to `inherit (self.packages.\${system}) docs;`
  so the check is literally the package output, no second import.

nix/docs.nix

@@ -90,18 +90,12 @@ let
   pickSubtrees =
     options: roots:
     let
-      walk =
-        path: tree:
-        if path == [ ] then
-          lib.getAttrFromPath path options
-        else
-          lib.setAttrByPath path (lib.getAttrFromPath path tree);
       pick =
         path:
-        let
-          exists = lib.hasAttrByPath path options;
-        in
-        if exists then lib.setAttrByPath path (lib.getAttrFromPath path options) else { };
+        if lib.hasAttrByPath path options then
+          lib.setAttrByPath path (lib.getAttrFromPath path options)
+        else
+          { };
     in
     lib.foldl' lib.recursiveUpdate { } (map pick roots);