Put back TODO

I don't think the `narHash` is in need of documentation more than the
other undocumented fields, but regardless this change has nothing to do
with that field and so we should leave the comment as is.

src/libstore/path-info.hh

@@ -29,6 +29,7 @@ struct ValidPathInfo
 {
     StorePath path;
     std::optional<StorePath> deriver;
+    // TODO document this
     Hash narHash;
     StorePathSet references;
     time_t registrationTime = 0;