crypto/sha512: update comment for the Sum512 function.
crypto/sha512: update comment for the Sum512 function.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/10660043

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

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