Re: Undocumented hyperlinks in doc strings.

[Richard Stallman]

    The only way I can think of that won't make newcomment.el's code
    even more complex (and significantly more ugly) is to accept
    expressions (rather than just numbers) for comment-add.

Using user-specified expressions is ugly, and we would have to mark
the variable risky, which is undesirable for an existing option
that was not risky before.

It would be better to add a new variable comment-add-function
and use that, if non-nil, rather than comment-add.