chore: update built-in metrics description

[heitortsergent]

What?

Update some of the descriptions for the Web Vital Metrics, and reorder the tables on that page.

Checklist

Please fill in this template:

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the make docs command locally and verified that the changes look good.

If updating the documentation for the most recent release of k6:

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant (e.g. when correcting a documentation error) folders of the previous k6 versions of the documentation.

[ppcano]

Two changes have been requested.

Additionally, there are conflicting files to be resolved before this can be merged.

I am unsure whether it is necessary to update old API versions.

[heitortsergent]

Thanks for the review @ppcano 🙇 .