[spec/type] Improve Implicit Conversions section

[ntrel]

Fixes #4264.

Fix backwards noreturn implicit conversion item (oops).
Document qualified array/pointer element type needing qualified void element type.
Add example.
Add link to qualifier conversions.

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[dkorpel]

Fix backwards noreturn implicit conversion item (oops).

I always have to stop and think about that, it's one of those things to very easily get backwards :)

[dkorpel]

This is going to raise some eyebrows, but the spec is there to describe semantics, not idiomatic code. I'm considering adding a bit of a rationale section at some point.