@babel/preset-env: Clarify `useBuiltIns` usage and purpose

[bowernite]

This PR clarifies what useBuiltIns is for, and how to use it, better (IMO 🙂).

Specifically, I really struggled using this option in one of my projects. It took me a long time to understand:

• That this option only told Babel how to import polyfills, and that it didn't come with them
  • Specifically, that you had to npm install them yourself
• core-js and regenerator/regenerator-runtime were exactly the two polyfills it supported
  • The old documentation focuses on core-js, and brushes over regenerator
• regenerator-runtime/runtime was installed via the regenerator npm package
  • Just took me a while to figure which exact package to install (regenerator-runtime? Just regenerator?)

Feedback welcome 😃

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[nicolo-ribaudo]

Could we also add a note that it is incompatible with transform-runtime? (ref: https://github.com/babel/babel/issues/10271)

[patrick-mcdougle]

Could we also add a note that it is incompatible with transform-runtime? (ref: babel/babel#10271)

@nicolo-ribaudo But if preset-env and transform-runtime don't work together, won't people lose the benefit of sharing the runtime/helper code across the entire project? Not sure people are going to like that.

[nicolo-ribaudo]

Sorry, I meant with transform-runtime's core-js option

[bowernite]

Could we also add a note that it is incompatible with transform-runtime? (ref: babel/babel#10271)

Do you have wording in mind on this? Might be easier than me trying to come up with something and having it be inaccurate 😅