Documentation, add kernel-parameters.txt entry for dis_ucode_ldr

dis_ucode_ldr was introduced in 65cef131 ("x86, microcode: Add a disable
chicken bit") and will disable microcode loading on x86.  This kernel
parameter is buried in the code and should be added to the Documentation.
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-parameters.txt

@@ -910,6 +910,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Disable PIN 1 of APIC timer
 			Can be useful to work around chipset bugs.
 
+	dis_ucode_ldr	[X86] Disable the microcode loader.
+
 	dma_debug=off	If the kernel is compiled with DMA_API_DEBUG support,
 			this option disables the debugging code at boot.