edac: Improve the comments to better describe the memory concepts

The Computer memory terminology has changed with time since EDAC was
originally written: new concepts were introduced, and some things have
different meanings, depending on the memory architecture.

Improve the definition of all related terms.

Also, describe each memory type in a more detailed fashion.

No functional changes. Just comments were touched.

Acked-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>