Correct build-dir error in manual, link relevant settings

This fixes out-of-date information that is no longer true, and makes the
up-to-date information more accessible.

(cherry picked from commit c5fa5e503a)

src/libstore/include/nix/store/globals.hh

@@ -776,6 +776,8 @@ public:
         "build-dir",
         R"(
             Override the `build-dir` store setting for all stores that have this setting.
+
+            See also the per-store [`build-dir`](@docroot@/store/types/local-store.md#store-local-store-build-dir) setting.
         )"};
 
     Setting<PathSet> allowedImpureHostPrefixes{

src/libstore/include/nix/store/local-store.hh

@@ -61,6 +61,8 @@ private:
             > `build-dir` must not be set to a world-writable directory.
             > Placing temporary build directories in a world-writable place allows other users to access or modify build data that is currently in use.
             > This alone is merely an impurity, but combined with another factor this has allowed malicious derivations to escape the build sandbox.
+
+            See also the global [`build-dir`](@docroot@/command-ref/conf-file.md#conf-build-dir) setting.
         )"};
 public:
     Path getBuildDir() const;