Ability to add private documentation to references

Right now all the godoc comments for references are added to the Kommentaar output. Sometimes it might be useful to have just some internal/private notes.

I can't really think of an elegant way of doing that. Only way I can think of is something along the lines of:

// Foo is a bar.
//
// {private}
// Not in Kommentaar output.
type Foo struct {
    // Public stuff.
    //
    // {private}
    // This is not in the Kommentaar output.
    Name
}

but I can't say I'm especially impressed by that. Alternatives to {private} might be ---, or something else.

Need to think about it. Suggestions welcome.

Start by reviewing the proposed private-comment syntax and the unresolved suggestions in this issue. Decide how private notes should be marked and how they should affect Kommentaar output for references, then define tests that demonstrate private notes are excluded while public godoc comments remain.