doc: PG 16 relnotes, misc updates

doc:  PG 16 relnotes, misc updates

*  document to_reg* accepting OIDs
*  document pg_log_standby_snapshot()
*  document pg_input_is_valid() and pg_input_error_info()
*  handle rename of function to pg_split_walfile_name()
*  fix character encoding problem for Przemyslaw Sztoch
*  remove partition section

Reported-by: jian he, Tom Lane, Bertrand Drouvot