Doc: Add example documentation

Adds minimal documentation for examples and ensures
that Mac Extras examples are shown in Qt Creator's
example list.

Task-number: QTBUG-33682
Change-Id: I6ba88bc7a1b22788d19d66d7d79dd486cd5add8c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: Iecbe3b7f6f6d75e615155c4103e0fa269ca6d3d2