x86/entry: Add some paranoid entry/exit CR3 handling comments

Andi Kleen was just asking me about the NMI CR3 handling and why
we restore it unconditionally.  I was *sure* we had documented it
well.  We did not.

Add some documentation.  We have common entry code where the CR3
value is stashed, but three places in two big code paths where we
restore it.  I put bulk of the comments in this common path and
then refer to it from the other spots.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: luto@kernel.org
Cc: bp@alien8.de
Cc: "H. Peter Anvin" <hpa@zytor.come
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20181012232118.3EAAE77B@viggo.jf.intel.com