Update the description of type of check_option reloption in docs.
Commit
773df883e8f7 changed the type of check_option reloption
from string to enum. But it forgot to update the description of
the type in the documentation.
Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYFvHF4n6yxF390YZgr4Q0Z0c2w0ihu=DLb8ipNOnNcqzQ@mail.gmail.com