Improve ShaHash.String comment and implementation.
Improve ShaHash.String comment and implementation.

The hash is not displayed big endian, but rather as a byte-reversed
hash as a hexidecimal string, so document it as such.

While here, speed up the function.  Some benchmarks:

BenchmarkShaHashString   2000000               996 ns/op
BenchmarkShaHashStringOld         100000             22701 ns/op

cqswxj2013/btcd,ppcsuite/ppcd-old,dajohi/btcd,jrick/btcd,aakselrod/btcd,DanielKrawisz/btcd,tb00/btcd,jcmincke/dcrd,Cleawing/btcd,soapboxsys/ombfullnode,tb00/btcd,toddfries/btcd,adiabat/btcd,hectorj/btcd,Stash-Crypto/btcd,cqswxj2013/btcd,davecgh/btcd,dajohi/btcd,btcsuite/btcd,davecgh/btcd,glv2/ppcd,ppcsuite/ppcd,qchain/btcd,ltcsuite/ltcd,glv2/ppcd,1d4Nf6/btcd,jrick/dcrd,ltcsuite/ltcd,pkothbauer/btcd,jimmysong/btcd,hectorj/btcd,decred/dcrd,wallclockbuilder/btcd,mathiasrw/btcd,jrick/dcrd,soapboxsys/ombfullnode,wallclockbuilder/btcd,decred/dcrd,qchain/btcd,ppcsuite/ppcd-old,runeaune/btcd,danoctavian/btcd,jrick/btcd,ppcsuite/ppcd,Dirbaio/btcd,Cleawing/btcd,btcsuitereleases/btcd,jongillham/btcd,mathiasrw/btcd,CryptocurrencyCabal/htcd,Roasbeef/btcd,jcmincke/dcrd,dan-da/btcd,FactomProject/btcd,BitGo/prova,stevenroose/btcd,CryptocurrencyCabal/htcd,runeaune/btcd,pkothbauer/btcd,Stash-Crypto/btcd,Dirbaio/btcd,Roasbeef/btcd,FactomProject/btcd,danoctavian/btcd,Dirbaio/btcd,BitGo/prova,dajohi/btcd,cjepson/btcd,jimmysong/btcd,btcsuitereleases/btcd,dan-da/btcd,cjepson/btcd,jrick/btcd,adiabat/btcd,1d4Nf6/btcd,toddfries/btcd,btcsuite/btcd,stevenroose/btcd,aakselrod/btcd,jongillham/btcd,jcmincke/dcrd,DanielKrawisz/btcd