Just a slip of the pen in the table "Function Description", in tha string: "setval ( regclass, bigint [, boolean ] ) → bigint", i think that something incorrect with the second comma. Thanks!
I don't see anything wrong. The square brackets show that this (3rd) parameter is optional. If you use three parameters then you need a comma between the 2nd and 3rd.
It's common to see [, parameter ] in function definitions. Look in text functions for many similar examples.