examples: adapted dataflow example for new semantics

doc/examples/dataflow/dataflow_producer-snippet.h

@@ -3,10 +3,14 @@ class Producer {
  public:
   explicit Producer(int seed) : seed_(seed), count_(4) {}
   bool Run(T& x) {
-    // produce a new value x
-    x = SimpleRand(seed_);
-    count_--;
-    return count_ >= 0;
+    if (count_ >= 0) {
+      // produce a new value x
+      x = SimpleRand(seed_);
+      count_--;
+      return true;
+    } else {
+      return false;
+    }
   }
 
  private:

doc/examples/dataflow/dataflow_source_function-snippet.h

 bool SourceFunction(std::string & str) {
-  std::getline(file, str);
-  return !file.eof();
+  if (!file.eof()) {
+    std::getline(file, str);
+    return true;
+  } else {
+    return false;
+  }
 }