UBUNTU: SAUCE: x86/speculation: Fix SSB command line documentation

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

Add info about the default mitigation which is part of upstream commit
f21b53b2 ("x86/speculation: Make "seccomp" the default mode for
Speculative Store Bypass").

Fixes: 37e59563 ("UBUNTU: SAUCE: x86/speculation: Make "seccomp" the default mode for Speculative Store Bypass")
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Documentation/kernel-parameters.txt

@@ -3880,6 +3880,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Not specifying this option is equivalent to
 			spec_store_bypass_disable=auto.
 
+			Default mitigations:
+			X86:	If CONFIG_SECCOMP=y "seccomp", otherwise "prctl"
+
 	spia_io_base=	[HW,MTD]
 	spia_fio_base=
 	spia_pedr=