qdoc: Detect and warn if snippet markers are more indented than snippet contents

QTBUG-138492 was caused by the following phenomenon.

This source code:

    //! [0]
int MyClass::myFunc()
{
    return 42;
}
    //! [0]

...would get rendered in the qdoc output like this:

int MyClass::myFunc()
{
return 42;
}

Suggestion
Detect and warn of such cases, because the //! markers need to reduce their indentation levels.