Documentation/ABI: Mention retbleed vulnerability info file for sysfs

While reporting for the AMD retbleed vulnerability was added in

  6b80b59b ("x86/bugs: Report AMD retbleed vulnerability")

the new sysfs file was not mentioned so far in the ABI documentation for
sysfs-devices-system-cpu. Fix that.

Fixes: 6b80b59b

 ("x86/bugs: Report AMD retbleed vulnerability")
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220801091529.325327-1-carnil@debian.org