Add a note about preset-env browserlists behavior

[ylemkimon]

@babel/preset-env only uses Android, Chrome, Edge, Firefox, IE, iOS, Safari, Node versions for detecting which transforms to include:

https://github.com/babel/babel/blob/e875fc7bc11ffdb8d5176028ae2b336bf7455c6c/packages/babel-preset-env/src/targets-parser.js#L32-L42

and silently ignores other browsers:

https://github.com/babel/babel/blob/e875fc7bc11ffdb8d5176028ae2b336bf7455c6c/packages/babel-preset-env/src/targets-parser.js#L72-L76

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[existentialism]

@ylemkimon let's update this section to be more explicit:

Example environments: chrome, opera, edge, firefox, safari, ie, ios, android, node, electron.

https://github.com/babel/website/blob/master/docs/preset-env.md#targets

[ylemkimon]

@existentialism I've moved the note to Targets section.

[ylemkimon]

Now, I realize opera is missing from browserNameMap in preset-env. I'll send a PR to babel/babel.