x86/microcode: Document the three loading methods

Paul Menzel recently asked how to load microcode on a system and I realized
that we don't really have all the methods written down somewhere.

Do that, so people can go and look them up.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170724101228.17326-3-bp@alien8.de


[ Fix whitespace noise in the new description. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>