Improve LCP and CLS DIAGNOSTICS text

**Feature request summary**
Improve the LCP and CLS DIAGNOSTICS text. Currently, the LCP diagnostic "Largest Contentful Paint element" leads to "[developer.chrome.com/docs/lighthouse/performance/lighthouse-largest-contentful-paint/](https://developer.chrome.com/docs/lighthouse/performance/lighthouse-largest-contentful-paint/)" which explains the metrics but not the impact that field data have on it. Since LCP and CLS are valid with field data, I suggest improving the text to paint a more precise insight into field vs lab data;
"The loading of other pages may influence the metrics of origin(aggregate data). Learn the difference between field and lab data."

**What is the motivation or use case for changing this?**
Improve UX and reduce GitHub issues for LCP and CLS about variability.

**How is this beneficial to Lighthouse?**
Improve UX.