doc/go1.13: mention faster sync.Mutex/RWMutex/Once
doc/go1.13: mention faster sync.Mutex/RWMutex/Once

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/+/188479
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

nawawi/go,golang/go,akutz/go,CAFxX/go,golang/go,golang/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,akutz/go,golang/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go