Improve visibility of `useBuiltIns: "entry"` on `@babel/polyfill` documentation

The [docs for `@babel/polyfill`](https://babeljs.io/docs/en/babel-polyfill) have some easily copy-paste-able lines that show how to directly import `@babel/polyfill`, but to get to the explanation of how to pass `useBuiltIns: "entry"`, you have to click to another page.

Anecdotally, some people might be using `@babel/polyfill` directly without `useBuiltIns`, which is unfortunate, as it significantly increases bundle size.

I'm wondering, would it make sense to put the full explanation about how to use the `useBuiltIns: "entry"` option on the `@babel/polyfill` page, to reduce the risk that people will forget to include it?

(The best solution in terms of code size would be to tell everyone to use the `"usage"` option--is there a particular reason that's still experimental?)

Start with the @babel/polyfill documentation page linked in the issue and follow its current link to the useBuiltIns: "entry" explanation. Review how the option is presented and whether the experimental "usage" question affects scope. Done means the relevant guidance is visible where users copy the direct import instructions and clearly explains the bundle-size implications.

Written by the indexing model from the issue text.