UBUNTU: SAUCE: Fix up IBPB and IBRS kernel parameters documentation

CVE-2018-3639 (x86)

There's no Documentation/admin-guide/ in 4.4 ...

Fixes: df043b74 ("x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature")
Signed-off-by: Juerg Haefliger <juergh@canonical.com>

Documentation/kernel-parameters.txt

@@ -2439,6 +2439,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable non-executable mappings
 
+	noibrs		[X86]
+			Don't use indirect branch restricted speculation (IBRS)
+			feature when running in secure environment,
+			to avoid performance overhead.
+
+	noibpb		[X86]
+			Don't use indirect branch prediction barrier (IBPB)
+			feature when running in secure environment,
+			to avoid performance overhead.
+
 	nosmap		[X86]
 			Disable SMAP (Supervisor Mode Access Prevention)
 			even if it is supported by processor.