docs-rst: admin-guide: add documentation for EDAC

EDAC is part of the Kernel's RAS facilities, with is useful for
system admins to detect errors. So, add it to the admin's guide.

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

Documentation/admin-guide/index.rst

@@ -59,6 +59,7 @@ configure specific aspects of kernel behavior to your liking.
    binfmt-misc
    mono
    java
+   ras
 
 .. only::  subproject and html