UefiCpuPkg/MpInitLib: expand comment on initial AP enumeration
    
    Before adding another AP enumeration mode, clarify the documentation on
    the current logic. No functional changes.
    
    Cc: Eric Dong <eric.dong@intel.com>
    Cc: Ray Ni <ray.ni@intel.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1515
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Ray Ni <ray.ni@intel.com>