kdf: `Kdf` trait documentation improvements (#2211) #2687

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	19s
test (stable)	18s
build (stable, wasm32-unknown-unknown)	14s
test (1.85.0)	21s
build (1.85.0, thumbv7em-none-eabi)	16s
build (stable, thumbv7em-none-eabi)	11s
	1m 39s

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