improve comment

Initial speed on i7-8665U

> go test -bench=. -test.cpu=1
goos: linux
goarch: amd64
pkg: github.com/henrydcase/nobs/hash/sha3
BenchmarkPermutationFunction 	 1634836	       732 ns/op	 273.18 MB/s
BenchmarkSha3_512_MTU        	   78438	     15340 ns/op	  88.00 MB/s
BenchmarkSha3_384_MTU        	  108807	     11025 ns/op	 122.45 MB/s
BenchmarkSha3_256_MTU        	  136902	      8767 ns/op	 153.98 MB/s
BenchmarkSha3_224_MTU        	  143377	      8355 ns/op	 161.57 MB/s
BenchmarkShake128_MTU        	  163569	      7108 ns/op	 189.94 MB/s
BenchmarkShake256_MTU        	  156534	      7643 ns/op	 176.64 MB/s
BenchmarkShake256_16x        	   10000	    112109 ns/op	 146.14 MB/s
BenchmarkShake256_1MiB       	     204	   5877014 ns/op	 178.42 MB/s
BenchmarkSha3_512_1MiB       	     100	  10967026 ns/op	  95.61 MB/s
PASS
ok  	github.com/henrydcase/nobs/hash/sha3	13.855s

hash/sha3/sha3.go

@@ -49,7 +49,8 @@ type state struct {
 	state     spongeDirection // whether the sponge is absorbing or squeezing
 }
 
-// BlockSize returns the rate of sponge underlying this hash function.
+// BlockSize returns block size in bytes. Corresponds to the input
+// block size B of the HMAC
 func (d *state) BlockSize() int { return d.rate }
 
 // Size returns the output size of the hash function in bytes.

hash/sha3/shake.go

@@ -4,9 +4,8 @@
 
 package sha3
 
-// This file defines the ShakeHash interface, and provides
-// functions for creating SHAKE and cSHAKE instances, as well as utility
-// functions for hashing bytes to arbitrary-length output.
+// SHAKE128 and SHAKE256 are FIPS approved XOFs. The cSHAKE128/256
+// are SHAKE-based XOFs supporting domain separation.
 //
 //
 // SHAKE implementation is based on FIPS PUB 202 [1]