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>