x86/cpufeature: Document cpu_feature_enabled() as the default to use

cpu_feature_enabled() should be used in most cases when CPU feature
support needs to be tested in code. Document that.

Reported-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20241031103401.GBZyNdGQ-ZyXKyzC_z@fat_crate.local