Documentation/x86: Document what /proc/cpuinfo is for

This has been long overdue. Write down what x86's version of
/proc/cpuinfo is and should be used for.

With improvements by dhansen.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20231129101700.28482-1-bp@alien8.de