sync: include links to the Go memory model in package documentation
The lack of links to https://go.dev/ref/mem in the sync package
documentation makes it difficult to read for people who have no previous
knowledge of that page. This PR includes the links where needed.
Fixes #67891
Change-Id: I0e1344cc6d7b702f4cb2e55fe0fcee3eb089391a
GitHub-Last-Rev:
427cf58aaeaae2e4b060248dd592e5fe8c6b7df4
GitHub-Pull-Request: golang/go#67892
Reviewed-on: https://go-review.googlesource.com/c/go/+/591395
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>