docs: tweak rsync flags for moving existing home dir to systemd-homed

docs: tweak rsync flags for moving existing home dir to systemd-homed

The documentation on moving an existing homedir into a systemd-homed managed
one suggests using rsync(1) with a bunch of flags to preserve as much metadata
as possible: permissions, xattrs, timestamps, etc. The previously suggested
flags were:

    rsync -aHAXv --remove-source-files …

… which does include mtimes, but not ctimes and atimes, because -a does not
include those:

    --archive, -a            archive mode is -rlptgoD (no -A,-X,-U,-N,-H)

This change adds the -N and -U flags to preserve even more file timestamps,
turning the command into:

    rsync -aHANUXv --remove-source-files …

The new flags are:

    --crtimes, -N            preserve create times (newness)
    --atimes, -U             preserve access (use) times