This option switch supports a total of 8 values, as told by
set_plan_disabling_options() and the documentation, but this was not
reflected in the output generated by --help.

Author: Junwang Zhao
Discussion: https://postgr.es/m/CAEG8a3+pT3cWzyjzKs184L1XMNm8NDnoJLiSjAYSO7XqpRh_vA@mail.gmail.com
Backpatch-through: 10