lib/types.nix: Document that it should not be used
lib/types.nix: Document that it should not be used

Co-authored-by: Robert Hensing <1ffad3fb1ab24362ef5dcb21f3f88b4b050415ea@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <edf0856f6febb9f05344b3b3b0060e83cede9be8@tweag.io>

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