Replicate C++ docs from master to beta branch

Original PR #3074 by @dgquintas
diff --git a/examples/cpp/helloworld/greeter_client.cc b/examples/cpp/helloworld/greeter_client.cc
index bfb7c12..6cd8353 100644
--- a/examples/cpp/helloworld/greeter_client.cc
+++ b/examples/cpp/helloworld/greeter_client.cc
@@ -35,11 +35,8 @@
 #include <memory>
 #include <string>
 
-#include <grpc/grpc.h>
-#include <grpc++/channel.h>
-#include <grpc++/client_context.h>
-#include <grpc++/create_channel.h>
-#include <grpc++/security/credentials.h>
+#include <grpc++/grpc++.h>
+
 #include "helloworld.grpc.pb.h"
 
 using grpc::Channel;
@@ -54,17 +51,28 @@
   GreeterClient(std::shared_ptr<Channel> channel)
       : stub_(Greeter::NewStub(channel)) {}
 
+  // Assambles the client's payload, sends it and presents the response back
+  // from the server.
   std::string SayHello(const std::string& user) {
+    // Data we are sending to the server.
     HelloRequest request;
     request.set_name(user);
+
+    // Container for the data we expect from the server.
     HelloReply reply;
+
+    // Context for the client. It could be used to convey extra information to
+    // the server and/or tweak certain RPC behaviors.
     ClientContext context;
 
+    // The actual RPC.
     Status status = stub_->SayHello(&context, request, &reply);
+
+    // Act upon its status.
     if (status.ok()) {
       return reply.message();
     } else {
-      return "Rpc failed";
+      return "RPC failed";
     }
   }
 
@@ -73,6 +81,10 @@
 };
 
 int main(int argc, char** argv) {
+  // Instantiate the client. It requires a channel, out of which the actual RPCs
+  // are created. This channel models a connection to an endpoint (in this case,
+  // localhost at port 50051). We indicate that the channel isn't authenticated
+  // (use of InsecureCredentials()).
   GreeterClient greeter(
       grpc::CreateChannel("localhost:50051", grpc::InsecureCredentials()));
   std::string user("world");