x86: mce: Update X86_MCE description in x86/Kconfig

- Clarify that this config controls thermal throttling
reporting too
- Clarify the types of errors reported by machine checks
- Drop references to ancient CPUs.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>