SDP Legacy Upgrade Guide doc updates:
* Customer feedback incorporated. THANK YOU!
* Added new 'Upgrade systemd service files' section.
* Cleaned up some incomplete text.
* Fixed typo 'MAILLOST' -> 'MAILLIST'.
* Added notes to crontab handling to deal with change from
daily_backup.sh to daily_checkpoint.sh and removal of
Updated SDP Legacy Upgrade Doc:
* Retitled 'Broker Config Files - Nothing to do' to drop the
'Nothing to do', since broker config files need to be moved from
/p4/N/bin for really old SDP installs. Added doc on what to do
* Fixed section numbering/indention level for sections
'Instance P4Review Scripts' and 'Broker Config Files'.