doc/next: add release note for sync/Map.Clear
Fix https://go-review.git.corp.google.com/c/go/+/561456
For #61696
Change-Id: I573bd14cf0e5c41adcc8c2b9481e85c5792c2a82
GitHub-Last-Rev:
c6fce6d25f8c071c92926ec3341855ad2303b876
GitHub-Pull-Request: golang/go#65534
Reviewed-on: https://go-review.googlesource.com/c/go/+/561755
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>