Clarify documentation of `KeyType` #25

Job	Run time
cargo fmt	6s
cargo test (ubuntu-latest, stable)	18s
cargo test (ubuntu-latest, nightly)	1m 26s
cargo test (macos-latest, stable)	1m 0s
cargo test (macos-latest, nightly)	3m 22s
cargo test (windows-latest, stable)	1m 45s
cargo test (windows-latest, nightly)	2m 36s
	10m 33s

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