Copy Up from dev: fix opt_perforce_sdp_backup.sh hardcoding the premature
/opt/perforce/p4-sdp OS package path instead of /opt/perforce/helix-sdp
(same bug class as SDP-1394, a separate hardcoded literal it missed), plus
related stale comment fixes in mkdirs.sh/install_sdp.sh, plus catching up
the rest of the outstanding dev->main Copy Up backlog (ReleaseProcessOverview.md,
SampleStorageSetup-AWS.adoc, deprecated_files.txt).