edac.txt: Improve documentation, adding RAS introduction

The edac.txt assumes that the reader has already deep knowledge
on RAS features. However, this may not be the case. So, add an
introduction chapter explaining the main concepts that are used by
the EDAC subsystem and by other RAS drivers within the Kernel.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>