Doc: Remove section about how to start threads

- Incomplete: It doesn't talk about how to use a raw QThread, or
  QRunnable, or Qt Concurrent.
- Redundant: Its contents are already presented in QThread's class ref,
  and the line before this section links to the "Multithreading
  Technologies in Qt" overview page which provides a more complete intro

Also remove snippet markers that are no longer used.

Change-Id: I89b7bd72f10c8ffdfd9b7772e2493050aafc9c88
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

examples/widgets/tutorials/threads/hellothread/hellothread.cpp

@@ -44,10 +44,7 @@
  * demonstrates use of QThread, says hello in another thread and terminates
  */
 
-//! [1]
-// hellothread/hellothread.cpp
 void HelloThread::run()
 {
      qDebug() << "hello from worker thread " << thread()->currentThreadId();
 }
-//! [1]

examples/widgets/tutorials/threads/hellothread/hellothread.h

@@ -42,13 +42,12 @@
 #define HELLOTHREAD_H
 
 #include <QThread>
-//! [1]
-// hellothread/hellothread.h
+
 class HelloThread : public QThread
 {
     Q_OBJECT
 private:
     void run();
 };
-//! [1]
+
 #endif // HELLOTHREAD_H

examples/widgets/tutorials/threads/hellothread/main.cpp

@@ -41,7 +41,6 @@
 #include <QtCore>
 #include "hellothread.h"
 
-//! [1]
 int main(int argc, char *argv[])
 {
     QCoreApplication app(argc, argv);
@@ -51,4 +50,3 @@ int main(int argc, char *argv[])
     thread.wait();  // do not exit before the thread is completed!
     return 0;
 }
-//! [1]

src/corelib/doc/src/threads-basics.qdoc

@@ -179,42 +179,9 @@
 
     \section1 Qt Thread Basics
 
-    QThread is a very convenient cross platform abstraction of native platform
-    threads. Starting a thread is very simple. Let us look at a short piece of
-    code that generates another thread which says hello in that thread and then
-    exits.
-
-    \snippet ../widgets/tutorials/threads/hellothread/hellothread.h 1
-
-    We derive a class from QThread and reimplement the \l{QThread::}{run()}
-    method.
-
-    \snippet ../widgets/tutorials/threads/hellothread/hellothread.cpp 1
-
-    The run method contains the code that will be run in a separate thread. In
-    this example, a message containing the thread ID will be printed.
-    QThread::start() will call the method in another thread.
-
-    \snippet ../widgets/tutorials/threads/hellothread/main.cpp 1
-
-    To start the thread, our thread object needs to be instantiated. The
-    \l{QThread::}{start()} method creates a new thread and calls the
-    reimplemented \l{QThread::}{run()} method in this new thread. Right after
-    \l{QThread::}{start()} is called, two program counters walk through the
-    program code. The main function starts with only the GUI thread running and
-    it should terminate with only the GUI thread running. Exiting the program
-    when another thread is still busy is a programming error, and therefore,
-    wait is called which blocks the calling thread until the
-    \l{QThread::}{run()} method has completed.
-
-    This is the result of running the code:
-
-    \code
-    //bad code
-    hello from GUI thread  3079423696
-    hello from worker thread  3076111216
-    \endcode
-
+    The following sections describe how QObjects interact with threads, how
+    programs can safely access data from multiple threads, and how asynchronous
+    execution produces results without blocking a thread.
 
     \section2 QObject and Threads