Docs: document the need for special configuration for symlinked pg_wal (#1818)

If an existing instance was configured with WAL residing outside of
PGDATA then currently a 'reinit' would lose such symlinks. So add some
bits of information on that to draw attention to this cornercase issue
and also add the --waldir option to the sample `postgresql.basebackup`
configuration sections to increase visibility.

Discussion: https://github.com/zalando/patroni/issues/1817

postgres1.yml

@@ -112,6 +112,7 @@ postgresql:
   basebackup:
       - verbose
       - max-rate: 100M
+#      - waldir: /pg-wal-mount/external-waldir # only needed in case pg_wal is symlinked outside of data_dir
   # Additional fencing script executed after acquiring the leader lock but before promoting the replica
   #pre_promote: /path/to/pre_promote.sh