pgsql: Fix doc oversight for custom WAL resource managers.

Fix doc oversight for custom WAL resource managers. Reported-by: Bharath Rupireddy Backpatch-through: 15 Discussion: https://postgr.es/m/calj2acu+at7rqnwezs59qsfg3zof4c4gsp7pt+pwilep0zr...@mail.gmail.com Branch -- master Details --- https://git.postgresql.org/pg/commitdiff/66af5eea31e06

pgsql: Fix doc oversight for custom WAL resource managers.

Fix doc oversight for custom WAL resource managers. Reported-by: Bharath Rupireddy Backpatch-through: 15 Discussion: https://postgr.es/m/calj2acu+at7rqnwezs59qsfg3zof4c4gsp7pt+pwilep0zr...@mail.gmail.com Branch -- REL_15_STABLE Details --- https://git.postgresql.org/pg/commitdiff/01b23d