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
$ python3 patronictl.py -c postgresql0.yml list
Error: Provided config file postgresql0.yml not existing or no read rights. Check the -c/--config-file parameter
Right now it's not really clear from --help what it does, flushing in software
context usually means persisting...so actually it's the opposite, so make
the intention more explicit.
