Doc: Fix more documentation warnings for Extending Qt Creator Manual

Change-Id: I3ba4e6d2f6cfa2c05c0ee7af6db70dcbb80ae725
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/coreplugin/ioutputpane.cpp

@@ -27,6 +27,8 @@
 
 #include "ioutputpane.h"
 
+namespace Core {
+
 /*!
     \class Core::IOutputPane
     \brief The IOutputPane class is an interface for providing \uicontrol Output panes.
@@ -258,3 +260,5 @@
     Displays \a number in the status bar button belonging to the output pane
     (for example, number of issues on building).
 */
+
+} // namespace Core