documentation: add troubleshooting erasure coded PGs section

Add a new section to the PG troubleshooting section that covers the most
common problems reported when an erasure coded pool fails to properly
map PGs to enough OSDs.

http://tracker.ceph.com/issues/10350 Fixes: #10350

Signed-off-by: Loic Dachary <ldachary@redhat.com>