I can never remember where to find these things either, so I do something like:
Select column_name, owner, table_name
Where column_name like '%COMMENT%'
and owner = 'SYS'
Order by column_name, owner, table_name;
That usually gives me a start . . . .
"The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous" - Gibbon, quoted by R.P.Feynman