doc: add return type

Signed-off-by: Darshan Sen <raisinten@gmail.com>

Author: RaisinTen

doc/api/worker_threads.md

@@ -1152,7 +1152,9 @@ added: REPLACEME
 
 > Stability: 1 - Experimental
 
-If true, the `Worker` object will keep the Node.js event loop active.
+* Returns: {boolean}
+
+If `true`, the `Worker` object will keep the Node.js event loop active.
 
 ### `worker.ref()`
 

test.js

@@ -0,0 +1 @@
+require('child_process').spawn('sleep', ['2'], ).unref();