Documentation: Add default kmemleak off case in kernel-parameters.txt

Add missing explanation about CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=y case.
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-parameters.txt

@@ -1583,6 +1583,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	kmemleak=	[KNL] Boot-time kmemleak enable/disable
 			Valid arguments: on, off
 			Default: on
+			Built with CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=y,
+			the default is off.
 
 	kmemcheck=	[X86] Boot-time kmemcheck enable/disable/one-shot mode
 			Valid arguments: 0, 1, 2