Simplify the wording of both.

Make the DecodeString docs more accurate:
DecodeString returns a slice, not a string.

Change-Id: Iba7003f55fb0a37aafcbeee59a30492c0f68aa4e
Reviewed-on: https://go-review.googlesource.com/115615Reviewed-by: Ian Lance Taylor <iant@golang.org>