docs: simplify DiamondRapids CPU docs This aligns the first line of the docs with the style used for previous CPU models, and simplifies the text in the remaining docs. Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org> Reviewed-by: Zhao Liu <zhao1.liu@intel.com> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>