Add #[must_comment] attribute for `*_unchecked` functions

[farnbams]

In fact, the reason why I pointed out that *_unchecked are not necessarily unsafe was a thread I opened before:

*_unchecked always unsafe?

Don't do it. Not unless you plan to write actual memory-unsafe code that relies on these types upholding these invariants. Questions like this are frequently asked, but the consensus is overwhelmingly that unsafe should be reserved for Undefined Behavior, period. Using it for other purposes cheapens its role in helping track down UB when it actually occurs. The _unchecked suffix ought to be quite sufficient for communicating the potential for logic errors.