add comments to keccak (#878) #6306

Job	Run time
pre-commit-x86_64	38s
pre-commit-arm64	49s
rust-checks-x86_64	40s
rust-checks-arm64	32s
rust-test-all-features-arm64	2m 57s
rust-doc	30s
rust-wasm-build-arm64	49s
rust-test-all-features-x86_64	3m 54s
rust-test-arm64	2m 50s
rust-wasm-build-x86_64	1m 3s
circuits-snapshot (sha512)	1m 30s
circuits-snapshot (sha256)	1m 31s
circuits-snapshot (zklogin)	1m 35s
circuits-snapshot (keccak)	1m 27s
circuits-snapshot (ethsign)	1m 35s
circuits-snapshot (hash_based_sig)	1m 31s
rust-test-x86_64	4m 1s
	27m 52s

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