UBUNTU: SAUCE: x86/speculation: Update 'mitigations=' documentation

BugLink: https://bugs.launchpad.net/bugs/1830176

The update to stable 4.4.180 added x86 Spectre v2 user mitigation,
so update the documentation accordingly.

From upstream commit d68be4c4 ("x86/speculation: Support
'mitigations=' cmdline option").
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Documentation/kernel-parameters.txt

@@ -2273,6 +2273,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 					       nobp=0 [S390]
 					       nospectre_v1 [PPC]
 					       nospectre_v2 [X86,PPC,S390]
+					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]
 					       l1tf=off [X86]
 					       mds=off [X86]