It has taken me a while to get to grips with \tl_set_rescan:Nnn <tl var>
{setup} {tokens}, not least the fact that *omission* from the setup
means "revert to usual catcode". I think this is worth documenting in
interface3.pdf, since it seems not unreasonable (at least it did to me)
to suppose that if one has explicitly changed a catcode using
\tl_set_rescan:Nnn, only a similarly explicit change would revert the
catcode to its usual value. In particular it would be helpful to
document the fact that using an empty setup { } reverts everything to
usual values.