Describe async behaviour of client libraries in page component README

[kwin]

The readme of the page component at https://github.com/adobe/aem-core-wcm-components/tree/main/content/src/content/jcr_root/apps/core/wcm/components/page/v3/page#loading-of-cssjs should be extended with a column explaining under which circumstances the according clientlib categories are loaded asynchronously (should describe both async and defer flag).