The following test on Linux shows the doc was not up-to-date for Linux:
```
var newAction = sigaction()
newAction.sa_flags = 0
sigemptyset(&newAction.sa_mask)
newAction.__sigaction_handler.sa_sigaction = { signal, siginfo, threadUserContext in
	print("got \(signal) from sigaction")
}
sigaction(15, &newAction, nil)
let s = DispatchSource.makeSignalSource(signal: 15)
s.setEventHandler{
	print("got \(s.data) signal 15 from dispatch")
}
s.activate()
usleep(5000)
kill(getpid(), 15)
sleep(1)
```

**Expected output**: `got signal` from both `sigaction` and `dispatch`.
**Actual output**: `got signal` from `dispatch` only.

If we run the test on macOS, we do get the signal from both sources.
If the dispatch source is not activated, we do get the signal from `sigaction` on Linux.

Tested on Docker, using the `swift:5.3.3` image and macOS 11.