The command nix-store --register-validity which is only referenced to in the documentation of exportReferencesGraph is completely undocumented https://nix.dev/manual/nix/2.26/language/advanced-attributes#adv-attr-exportReferencesGraph

This means that the file format that https://nix.dev/manual/nix/2.26/language/advanced-attributes#adv-attr-exportReferencesGraph generates is fully undocumented too.

We should document the nix-store --register-validity command.

Originally posted by @arianvp in #10279