Doc: Fix copy+paste errors in the QString class ref

Also bring text closer to the style guide at
http://qt-project.org/wiki/CppDocumentationStyle

Change-Id: I30b1c36ac125a10c002efeb36978ced0d7a8f8bf
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Add an overview page to compare multithreading techniques

Change-Id: I75e67ecb96423a3ebd82b32e6855378a73463fb7
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Doc: Move multithreading guidelines to the overview page

It's helpful to see how to choose among different solutions, right
after seeing short descriptions of all the solutions.

- Some minor rewording was done during the move
- The example about polling ports in a new thread was removed because
  there are better ways to do that without threads.

Change-Id: I2cb571a4dbf9be93fb0ec88c60fb7406996c345b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Doc: Remove page about how to start QThreads

Multiple issues:
- Incomplete: It doesn't talk about instantiating a raw QThread
- Redundant: Its contents are already presented in QThread's class ref
- Incorrect: It is legal to create a QThread before a QCoreApplication
- Irrelevant: The bit about QCoreApplication::exec() and the etymology
  of "GUI thread" is unrelated to the topic of starting threads

Also remove snippets that are no longer used

Change-Id: Ice1819845b5b2cf843719edaa7b0f4bbb1e8bd97
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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>