In default.conf cleaned up
all cases of config variables for secret values
i.e. have a generic explanation comment at the beginning
instead of several similar comments at each place

When VAR is for a secret value we must
also in case of VAR="${VAR:-default}" use
{ VAR="${VAR:-default}" ; } 2>/dev/null
otherwise when VAR is already set as VAR=secret
then with 'set -x' the plain VAR="${VAR:-default}"
would reveal the secret in the log as 'VAR=secret'

Keep the information that default.conf is sourced by usr/sbin/rear directly (using 'source') while {site,local}.conf are sourced using Source() which can 'set -x'.

In etc/rear/local.conf added an explanation how to set a secret value

Less duplicate expanation in local.conf - instead a generic reference to default.conf