This question has been asked before, but each time the accepted answer is simply a resignation to provide function descriptions using Application.MacroOptions (VBA6) (VBA7), but this information does not actually appear as a tooltip, so it does not solve my problem.
The Goal
What we all desire is to be able to define custom functions by any means (VBA, VSTO, or COM add-in) and give the user the benefit of a pop-up/tool-tip description of the function and its parameters, as appears for every built-in Excel function, either inline or in the formula bar:
The widely accepted answer to this need is that it is not possible for custom functions, but I wish to challenge that belief.
The Problem
Currently, the best I've seen anyone do is to define functions (often using the above MacroOptions call) so that when bringing up the function dialog (the fx button in the formula bar) their function and parameter descriptions appear as below:
As you can see, this is a complicated function with many parameters. If the user is not aware of this "function arguments" dialogue and how to bring it up, and are instead familiar only with Excel's standard tooltip, they will only see the formula name and no additional help: