Given the following code:
"
    private String testFunction()
    {
        final String testString = "Test";
        // comment at the end of introduce function
        return testString;
    }
"
Marking 
"
        final String testString = "Test";
        // comment at the end of introduce function
"
(The cursor is in the line of "return testString")
and pressing "Introduce Method" (Alt+Shift+M) the following code is generated:
"
    private String testFunction()
    {
        String testString = introducedMethod();
        // comment at the end of introduce function
        return testString;
    }

    private String introducedMethod()
    {
        final String testString = "Test";
        return testString;
    }
"
The comment should be in the introduced method, because it was in the marked area.

Just as a note: After the refactoring the text is marked in these lines:
"
        String testString = introducedMethod();
        // comment at the end of introduce function
        return testString;
    }

    private String introducedMethod()
    {
        final String testString = "Test";
"
To me, this doesn't make much sense. Maybe it would be better if there was no marked text after the refactoring.