Fix typo in tmpfs /etc/fstab entry which stopped it working in the doc.
Mention in pre-requisites for failover and failover guide the need to review
OS Config for your failover server.
Document Ubuntun 2020.04 LTS and CentOS/RHEL 8 support. Note performance
has been observed to be better with CentOS.
Document pull.sh and submit.sh in main SDP guide (remove from Unsupported doc).
Update comments in triggers to reflect that they are reference implementations, not
just examples. No code change.