The documentation mentioned the use of log_checkpoints, that cannot be
used in this context.  This commit replaces log_checkpoints with
force_parallel_mode, a developer option useful to perform checks related
to parallelism.

Oversight in 854434c5.

Author: Haiying Tang
Discussion: https://postgr.es/m/OS0PR01MB6113954B883ACEB2DDC973F2FBE59@OS0PR01MB6113.jpnprd01.prod.outlook.com
Backpatch-through: 14