Document the mutex shenanigans used to implement block until ready

bindings/go/src/fdb/futures.go

@@ -87,6 +87,13 @@ func fdb_future_block_until_ready(f *C.FDBFuture) {
 		return
 	}
 
+	// The mutex here is used as a signal that the callback is complete.
+	// We first lock it, then pass it to the callback, and then lock it
+	// again. The second call to lock won't return until the callback has
+	// fired.
+	//
+	// See https://groups.google.com/forum/#!topic/golang-nuts/SPjQEcsdORA
+	// for the history of why this pattern came to be used.
 	m := &sync.Mutex{}
 	m.Lock()
 	C.go_set_callback(unsafe.Pointer(f), unsafe.Pointer(m))