Documentation for the decode function has an incorrect reference

The documentation for the decode function introduced in SPARK-33527 refers erroneously to Oracle. It appears that the documentation string has been in large parts copied from https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/DECODE.html#GUID-39341D91-3442-4730-BD34-D3CF5D4701CE

This proposes to update the documentation for the decode function to fix the issue.