Add docs for preset-env's modules:auto behavior

[loganfsmyth]

We didn't document babel/babel#8485

[mi-yu]

Hi, I'd like to pick this up if nobody else already has, but I have a few questions:

• is the auto option present in all current versions of Babel, or only 7.x?
• is the documentation here a good starting point to put on the website docs?

[existentialism]

@mi-yu only 7.x, and yeah, augmenting https://babeljs.io/docs/en/babel-preset-env#modules with explaining false, a specific module type, and auto is what we need!

[mi-yu]

Thanks!

[lidoravitan]

is it still relevant?

[franz-ka]

i also think that the documentation for this option should be more clear as there are lots of tutorials out there saying that you should set modules: false to make use of code splitting or tree shaking. maybe there are lots of people corcerned if these features are actually working or not in their bundles. i would add in the documentation that for most of the cases "auto" defaults to false and mybe specify when it does not.