API dox: fix missing note to call setXMLFile with KParts::MainWindow

Summary:
Calling KXmlGuiWindow::setupGUI(flags, xmlfile) without the "Create"
flag will result in the xmlfile argument being ignored.
So setXMLFile(xmlfile) needs to be explitly called.

Also

• adds some @c markup for code snippets
• fixes "Default flag" -> "Default flag set"
• adds KXmlGuiWindow::Create to links for warning, as it has the used info

Reviewers: elvisangelaccio, Frameworks, dfaure

Reviewed By: dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5181