sync: update comments for Once.done
Sync with CL 666895.
Change-Id: I49c4a7f88d87cee9c30a858facd3cd8348efdf94
GitHub-Last-Rev:
88ac1c9c4131aa3f5dfb9c7923e49e46808c409d
GitHub-Pull-Request: golang/go#76026
Reviewed-on: https://go-review.googlesource.com/c/go/+/714360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>