Edac: Add ABI Documentation for the new device nodes

The EDAC ABI were extended to add support for per-DIMM or per-rank
information and silkscreen labels. Properly document them.

Most of the comments there came from edac.txt descriptions of the
fields that are part of the legacy csrowX ABI (e. g.
/sys/devices/system/edac/mc/mc*/csrow*/*).

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>