Mention faster sync.Mutex/RWMutex/Once in the 1.13 release notes.

Change-Id: I29d8a5004a0af42542e8db82a8c9e2e06a15dbb0
GitHub-Last-Rev: 2995401dab563ea5af98c0f5351f51a6116f105e
GitHub-Pull-Request: golang/go#33404
Reviewed-on: https://go-review.googlesource.com/c/go/+/188479Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>