The Editions and Functionality matrix refers to custom
parameter functions, such as Sunday, Monday, etc. last week. Where is it documented how to set up custom parameters?

Custom parameter functions are used to modify the report's record selection formula locally. To access this feature, go to Properties (of a schedule) - Report and click the Advanced button. The saved record selection formula will be shown. Place the cursor at the part of the formula you wish to modify and click Insert. You will be able to insert the parameter functions - or create your own functions. More information on using the Insert feature can be found in the help file under "Insert".