sync: refer to Locker interface methods in RWMutex.RLocker doc
The methods being implemented are part of the Locker interface,
not the RWMutex struct.
Fixes #68250.
Change-Id: I609c4d5c44e90a12914a8678971ba295519cc265
Reviewed-on: https://go-review.googlesource.com/c/go/+/595875
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Than McIntosh <thanm@google.com>