Update instructions for manual mdadm start, drop --auto=yes --symlink=no from swraid

The "--auto=yes --symlink=no" options shouldn't be relevant any
longer (symlink actually doesn't seem to do what it's supposed).

The "mdmonitor" service doesn't do what we thought it does,
so instead recommend usage of "mdadm --assemble --scan" instead.

Related to grml/grml#45