context: document CancelFunc to be safe for simultaneous use by multiple goroutines

Fixes #32145

Change-Id: If4c9dd3a2af748974141ad6e571f80efcbaad772
Reviewed-on: https://go-review.googlesource.com/c/go/+/177899Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/context.go

@@ -220,6 +220,7 @@ func TODO() Context {
 
 // A CancelFunc tells an operation to abandon its work.
 // A CancelFunc does not wait for the work to stop.
+// A CancelFunc may be called by multiple goroutines simultaneously.
 // After the first call, subsequent calls to a CancelFunc do nothing.
 type CancelFunc func()