Specify applicability and the default value. Also state that, in case of
EFI, the microcode update blob specified in the EFI cfg takes precedence
over `ucode=scan`, if the latter is specified on Xen commend line.
No functional changes.
Signed-off-by: Eslam Elnikety <elnikety@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>