sync/atomic: amend comments for Value.{Swap,CompareAndSwap}
Related to CL 241661, CL 403094.
Change-Id: I86877d9a013b05b1a97f9aa1333cd96ce98469cb
GitHub-Last-Rev:
7fedca5848cb17730d4ccb586dc03cfcb1b83259
GitHub-Pull-Request: golang/go#76259
Reviewed-on: https://go-review.googlesource.com/c/go/+/719660
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>