builtin: document appending a string to a byte slice Fixes issue 4873.
builtin: document appending a string to a byte slice
Fixes issue 4873.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7421043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go