Maybe: "Deprecate `\c_novalue_tl` in favor of `\NoValue` or its expansion"

[304HoeJane]

Maybe: "Deprecate \c_novalue_tl in favor of \NoValue or its expansion"

Originally posted by @Skillmon in https://github.com/latex3/latex3/pull/1806#discussion_r2433884369

[Skillmon]

@304HoeJane that comment of mine no longer applies. You can still use the one-step expansion of \c_novalue_tl, but with the current implementation you must not o-expand \NoValue further (and since this doesn't break backwards compatibility, which only needs to cover o, e, and x type expansion of \c_novalue_tl that's totally fine).