x86/microcode: Document the whole late loading problem

Commit

  d23d33ea ("x86/microcode: Taint and warn on late loading")

started tainting the kernel after microcode late loading.

There is some history behind why x86 microcode started doing the late
loading stop_machine() rendezvous. Document the whole situation.

No functional changes.

  [ bp: Fix typos, heavily massage. ]

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220813223825.3164861-2-ashok.raj@intel.com