Bug#871490: mention how to force listing of /var/lib/systemd/coredump

[積丹尼 Dan Jacobson]

# journalctl |grep -c 'Processing coredump'
2
# coredumpctl --no-legend | wc -l
2
# ls /var/lib/systemd/coredump/ | wc -l
12

So on the coredumpctl, after

           It's worth noting that different restrictions apply to data saved
           in the journal and core dump files saved in
           /var/lib/systemd/coredump, see overview in systemd-coredump(8).
           Thus it may very well happen that a particular core dump is still
           listed in the journal while its corresponding core dump file has
           already been removed.

it should also say

           And it also might very well happen that a particular core dump is not
           listed in the journal while its corresponding core dump file
           is still indeed sitting in /var/lib/systemd/coredump.

           That is because you didn't enable persistent logging in
           journald. See /usr/share/doc/systemd/README.Debian.gz , and
           in the meantime use ls(1) to see the coredumps made before
           the last boot.