I tried @ScottK's approach, first as a side feature of my functional UDF, then as a standalone _Help suffix version when I ran into trouble (see below). In hindsight, the latter approach is better anyway--more obvious to a user attentive enough to see a tool tip, and it doesn't clutter up the functional code.
I figured if an inattentive user just typed the function name and closed the parentheses while he thought it over, help would appear and he would be on his way. But dumping a bunch of text into a single cell that I cannot format didn't seem like a good idea. Instead, When the function is entered in a cell with no arguments i.e.
= interpolateLinear()
or
= interpolateLinear_Help()
a msgBox opens with the help text. A msgBox is limited to ~1000 characters, maybe it's 1024. But that's enough (barely 8^/) for my overly tricked out interpolation function. If it's not, you can always open a user form and go to town.
The first time the message box opened, it looked like success. But there are a couple of problems. First of course, the user has to know to enter the function with no arguments (+1 for the _Help suffix UDF).
The big problem is, the msgBox reopens several times in succession, spontaneously while working in unrelated parts of the workbook. Needless to say, it's very annoying. Sometimes it goes on until I get a circular reference warning. Go figure. If a UDF could change the cell formula, I would have done that to shut it up.
I don't know why Excel feels the need recalculate the formula over and over; neither the _Help standalone, nor the full up version (in help mode) has precedents or dependents. There's not an application.volatile statement anywhere. Of course the function returns a value to the calling cell. Maybe that triggers the recalc? But that's what UDFs do. I don't think you can not return a value.
Since you can't modify a worksheet formula from a UDF, I tried to return a specific string --a value --to the calling cell (the only one you can change the value of from a UDF), figuring I would inspect the cell value using application.caller on the next cycle, spot my string, and know not to re-display the help message. Seemed like a good idea at the time--didn't work. Maybe I did something stupid in my sleep-deprived state. I still like the idea. I'll update this when (if) I fix the problem. My quick fix was to add a line on the help box: "Seek help only in an emergency. Delete the offending formula to end the misery.
In the meantime, I tried the Application.MacroOptions approach. Pretty easy, and it looks professional. Just one problem to work out. I'll post a separate answer on that approach later.