cpp: do not mention -1 for command_port option

-1 is not a valid port number and thus do not mean "no gRPC command server".

If you try to pass -1 to command_port you get:

$ bazel --command_port=-1 build //...
Invalid argument to --command_port: '-1'.
Must be a valid port number or 0.

Change-Id: I6c17167f6a285b21fcd064cea8bdc7e6770ac984
PiperOrigin-RevId: 155352835

src/main/cpp/startup_options.h

@@ -199,8 +199,7 @@ class StartupOptions {
   // empty string if it was not specified on the command line.
   std::string GetExplicitHostJavabase() const;
 
-  // Port for gRPC command server. 0 means let the kernel choose, -1 means no
-  // gRPC command server.
+  // Port to start up the gRPC command server on. If 0, let the kernel choose.
   int command_port;
 
   // Connection timeout for each gRPC connection attempt.