Is there any website or handbook to find all the custom functions that can be used in Kreporter?
Ralph, in customFunctions you can use any SQL function that is applicable for the field type. Date functions, string functions, number functions, even IF Statements are possible. Full select Queries may also be used. As documentation we therefore refer to MySQL reference manual.
adding to that make sure to read this document http://www.spicecrm.io/download/customizing-queries-in-kreporter/ as it is the base for any customizations of queries and also differentiates between the SQL and PHP functions.