edac.txt: add a section explaining the dimmX and rankX directories

Documentation for those are missing at the EDAC description.

I guess we end by moving such descriptions in the past to the
ABI document (or only added it there), but it means that the
EDAC documentation is incomplete. So, add it there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>