audit: documentation of audit= kernel parameter

Further documentation of the 3 possible kernel value of the audit
command line option.

Signed-off-by: Eric Paris <eparis@redhat.com>

Documentation/kernel-parameters.txt

@@ -465,6 +465,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	audit=		[KNL] Enable the audit sub-system
 			Format: { "0" | "1" } (0 = disabled, 1 = enabled)
+			0 - kernel audit is disabled and can not be enabled
+			    until the next reboot
+			unset - kernel audit is initialized but disabled and
+			    will be fully enabled by the userspace auditd.
+			1 - kernel audit is initialized and partially enabled,
+			    storing at most audit_backlog_limit messages in
+			    RAM until it is fully enabled by the userspace
+			    auditd.
 			Default: unset
 
 	audit_backlog_limit= [KNL] Set the audit queue size limit.