Clarify docs about env "useBuiltIns" behavior and common uses.

[AndrewSouthpaw]

I think a lot of people wind up at this section of the docs and can get pretty easily overwhelmed by all the details about polyfills, core-js, etc.

My intent was to provide some more language around how the system works, and what people "probably" will want to choose the majority of the time, to guide them in the right direction.

I also switched the order of the detailed sections on usage and entry so that it reflects the order of the options presented (usage, entry, false).

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