Document Oracle-specific functions (DECODE, NVL, LTRIM, RTRIM, SUBSTR, GREATEST, LEAST) in the SQL reference.
Same goes for MySQL-specific functions (e.g. JSON_TYPE).
I don't think we should have separate lists of Oracle-specific functions and MySQL-specific functions. Because quite a few functions appear in more than one place. Better, I think, to have a concise annotation against each function which tables it occurs in.
The current list of tables is standard, oracle, spatial, mysql. Perhaps also indicate whether a function is an extension to the SQL standard but still occurs in Calcite's default table.