The starting place should the Information Center topic on Database Routines. The Related Information topic under Database lists numerous other resources.
One listed resource that you might find particularly useful is the Stored Procedures, Triggers, and User-Defined Functions on DB2 Universal Database for iSeries Redbook.
And, of course, the actual SQL Reference manuals will always be needed for reference about the SQL statements themselves. (They can be downloaded from the Information Center.)
However, since the purpose is “cross-platform compatibility”, you might want to be familiar with how other platforms do things. You won’t find a lot of IBM info on how to port from DB2 to other RDBMSs — you’ll have to go to other vendors to see what porting guides they offer. But maybe the IBM DB2 for i Porting Information page will be helpful. It can help with how to port specifications from some others to DB2, so you might be able to work out how to go in the opposite directions.