Documentation/x86: Add documentation for /proc/cpuinfo feature flags

/proc/cpuinfo shows features which the kernel supports. Some of these
flags are derived from CPUID, and others are derived from other sources,
including some that are entirely software-based. Currently, there is
not any documentation in the kernel about how /proc/cpuinfo flags are
generated and what it means when they are missing.

Add documentation for /proc/cpuinfo feature flags enumeration.
Document how and when x86 feature flags are used. Also discuss what
their presence or absence mean for the kernel and users.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Co-developed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Kyung Min Park <kyung.min.park@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200831183500.15481-1-kyung.min.park@intel.com