Hi Davor,
The overall description for a function is not difficult and you should
choose your arguments wisely so that they are more or less self
explanatory.
The parameter cell will show up in the Function Wizard Help
without any intervention, so choose your parameter names
carefully. The description can be entered as follows: Alt+F11
(VBE), F2 (View, Object Browser), Select the module (or <globals>)
on left, select the function on right, RClick, properties, fill in description.
But it is actually possible to put descriptions on the individual
parameter with an addin from Laurent Longre.
Creating new function categories (Excel 97 and up) in the Function Wizard.
http://longre.free.fr/english/func_cats.htm
This addin also by Laurent Longre in the English subset
http://longre.free.fr/english of his main site which is in French.
Install Macros and User Defined Functions
Entering a Description for the Function [fx] Wizard
http://www.mvps.org/dmcritchie/excel/install.htm#fx