Rewrite MSI-HOWTO

I didn't find the previous version very useful, so I rewrote it.

Signed-off-by: Matthew Wilcox <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Reviewed-by: Grant Grundler <[email protected]>
Signed-off-by: Jesse Barnes <[email protected]>

Author: Matthew Wilcox