mdadm man page update proposed

Hi!

I believe it would be helpful to many to note in the mdadm manual
page, that only version 0.90 superblocks can be auto-detected by
current kernels. There is a notice at version 1 and up that "they have
some limitations", but this important fact isn't mentioned anywhere in
the man page.

Thanks a lot.

-- 
Nagy Attila Gabor