Missing max_parallel_degree configuration parameter in PostgreSql 9.6

I am not able to set max_parallel_degree.

Documentation: https://wiki.postgresql.org/wiki/Parallel_Query

Just now I updated my postgresql test database server from version 9.5.5 to 9.6. Server is restarted, up and running. According documentation I should be able to set parallel degree for actual user, but this parameter is not in configuration table.

When I try to set max_parallel_degree = 4; I get this:
ERROR: unrecognized configuration parameter "max_parallel_degree"

My configuration:
ubuntu server (updated), postgresql 9.6 in VirtualBox machine with 6 cores assigned.
My config file: /etc/postgresql/9.6/main/postgresql.conf does not contain rows with "parallel" word.
server_version = 9.6.1; psql version = 9.6.1

select * from pg_settings where name like '%par%';

Results:

debug_print_parse               | off
force_parallel_mode             | on  *
log_parser_stats                | off
max_parallel_workers_per_gather | 4   *
max_prepared_transactions       | 0
min_parallel_relation_size      | 1024
parallel_setup_cost             | 1000
parallel_tuple_cost             | 0.1

*This settings was set by me after run psql.

What can I do to enable parallel query running on PostgreSql 9.6?

Copyright Notice：Content Author:「Atiris」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/40490724/missing-max-parallel-degree-configuration-parameter-in-postgresql-9-6