doc: Move wal_init_zero and wal_recycle descriptions to proper section.
The group of wal_init_zero and wal_recycle is WAL_SETTINGS in guc.c,
but previously their documents were located in
"Replication"/"Sending Servers" section. This commit moves them to
the proper section "Write Ahead Log"/"Settings".
Back-patch to v12 where wal_init_zero and wal_recycle parameters
were introduced.
Author: Fujii Masao
Discussion: https://postgr.es/m/
b5190ab4-a169-6a42-0e49-
aed0807c8976@oss.nttdata.com