Consider `@noRd` for `assert_*` and any other non-exported functions

[brookslogan]

Even though these functions are not exported, they still will show up in the "Reference" with the exported functions. Assuming this is unintentional, we should add @noRd (no .Rd / compiled doc at all) or @keywords internal (documentation is compiled, but not listed in the "Reference" index); people seem to favor the first approach.