Fix opt_perforce_sdp_backup.sh hardcoding the premature /opt/perforce/p4-sdp
OS package path instead of /opt/perforce/helix-sdp.
This is the same bug class as SDP-1394 (install_sdp.sh and others shipped
premature /opt/perforce/p4-sdp path) but a separate, previously-missed
hardcoded literal that SDP-1394's original SDPPackageBase= search pattern
did not catch. opt_perforce_sdp_backup.sh independently hardcodes the
literal string "opt_perforce_p4-sdp.$ThisHost" for its backup/staging
directory names and tarball internal structure, both in -man usage text
and in the functional BackupDir/StagingDir/tar commands.
Confirmed live via a real Ubuntu 22 install + backup run: the backup
service reported "Backups are here: /mnt/p4depots/backup/opt_perforce_p4-sdp.<host>"
even on a fresh install using the already-fixed install_sdp.sh.
Also fixed 4 stale comment references (not functional, but same class)
in mkdirs.sh and install_sdp.sh that still described the package
structure as /opt/perforce/p4-sdp instead of /opt/perforce/helix-sdp,
including one that incorrectly said the required subdirectories were
"p4-sdp and downloads" instead of "sdp and downloads".
Regenerated script man pages and SDP_Guide.Unix.html (HTML only, no PDF,
per dev branch policy) to reflect the corrected paths.