Documentation/x86: Explain the state component permission for guests

Commit 980fe2fd ("x86/fpu: Extend fpu_xstate_prctl() with guest
permissions") extends a couple of arch_prctl(2) options for VCPU threads.
Add description for them.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Yang Zhong <yang.zhong@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20230121001900.14900-5-chang.seok.bae%40intel.com