sync: tidy WaitGroup documentation, add WaitGroup.Go example
This reframes the WaitGroup documentation with Go at its center and
Add/Done as more "advanced" features.
Updates #63796
Change-Id: I8101972626fdb00c6f7fb185b685227823d10db1
Reviewed-on: https://go-review.googlesource.com/c/go/+/662975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>