Documentation/x86: Explain the purpose for dynamic features

This summary will help to guide the proper use of the enabling model.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20230121001900.14900-2-chang.seok.bae%40intel.com