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: NBorislav Petkov <bp@alien8.de>
Signed-off-by: NTony Luck <tony.luck@intel.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Acked-by: NThomas 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

arch/x86/include/asm/intel-family.h

@@ -11,6 +11,21 @@
  * While adding a new CPUID for a new microarchitecture, add a new
  * group to keep logically sorted out in chronological order. Within
  * that group keep the CPUID for the variants sorted by model number.
+ *
+ * The defined symbol names have the following form:
+ *	INTEL_FAM6{OPTFAMILY}_{MICROARCH}{OPTDIFF}
+ * where:
+ * OPTFAMILY	Describes the family of CPUs that this belongs to. Default
+ *		is assumed to be "_CORE" (and should be omitted). Other values
+ *		currently in use are _ATOM and _XEON_PHI
+ * MICROARCH	Is the code name for the micro-architecture for this core.
+ *		N.B. Not the platform name.
+ * OPTDIFF	If needed, a short string to differentiate by market segment.
+ *		Exact strings here will vary over time. _DESKTOP, _MOBILE, and
+ *		_X (short for Xeon server) should be used when they are
+ *		appropriate.
+ *
+ * The #define line may optionally include a comment including platform names.
  */
 
 #define INTEL_FAM6_CORE_YONAH		0x0E