Documentation/ABI: Mention retbleed vulnerability info file for sysfs

While reporting for the AMD retbleed vulnerability was added in

  6b80b59 ("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: 6b80b59 ("x86/bugs: Report AMD retbleed vulnerability")
Signed-off-by: Salvatore Bonaccorso <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: carnil

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -523,6 +523,7 @@ What:		/sys/devices/system/cpu/vulnerabilities
 		/sys/devices/system/cpu/vulnerabilities/tsx_async_abort
 		/sys/devices/system/cpu/vulnerabilities/itlb_multihit
 		/sys/devices/system/cpu/vulnerabilities/mmio_stale_data
+		/sys/devices/system/cpu/vulnerabilities/retbleed
 Date:		January 2018
 Contact:	Linux kernel mailing list <[email protected]>
 Description:	Information about CPU vulnerabilities