Comment update.

Change-Id: If0d054216f9953f42df04647b85c38008b85b026
GitHub-Last-Rev: 133b4670be6dd1c94d16361c3a7a4bbdf8a355ab
GitHub-Pull-Request: golang/go#31539
Reviewed-on: https://go-review.googlesource.com/c/go/+/172700Reviewed-by: Austin Clements <austin@google.com>