I agree that this is a very important concept to document clearly, but I don't think the pinMode() reference is the appropriate place for it. Instead, I would put in on that digital pins tutorial which is so prominently linked to from the pinMode() reference page. The pinMode() reference page should be for documenting pinMode(), and only for documenting pinMode().
Unfortunately, although the Arduino Language Reference content is accessible for anyone to make proposals (though whether the powers that be at Arduino will ever bother to consider that proposal is another question), the library reference and tutorials content is hidden away (top secret stuff right?) sheltered from any chance of contributions from the community.