x86/cpu: Explain Intel model naming convention

Dave Hansen spelled out the rules in an e-mail:

 https://lkml.kernel.org/r/91eefbe4-e32b-d762-be4d-672ff915db47@intel.com

Copy those right into the <asm/intel-family.h> file to make it easy for
people to find them.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190815224704.GA10025@agluck-desk2.amr.corp.intel.com