core: Add comment on `ZeroizeOnDrop` #467

Job	Run time
Dusk Analyzer / Dusk Analyzer	16s
test cirucits / Run tests	3m 2s
Code Analysis / Clippy Check	22s
Code Analysis / Rust Format	7s
test cirucits no-default-features / Run tests	12s
test core / Run tests	11s
test core rkyv compiles / Run tests	13s
	4m 23s

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