Update preset-env.md

[epreston]

Clarify exception to "targets" handling for backward compatibility.

babel/babel#15371 (comment)

Closes babel/babel#15371

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[epreston]

This clarifies the behaviour of the targets setting in @babel/preset-env. The behaviour of esmodule and browsers "intersection" differs depending on where the settings come from.

• from the .browserslistrc - intersection
• from the top level targets setting - intersection
• from the local targets setting within the @babel/preset-env option - no intersection

Further detail is provided in the github issue: babel/babel#15371