Add section on nested/multiline ternaries

[ljharb]

Specifically, prohibiting nested ternaries (to match https://github.com/airbnb/javascript/blob/master/packages/eslint-config-airbnb/rules/style.js#L63-L64) and perhaps discouraging multiline ternaries (not sure if there's a rule for it).

[bgits]

opened a PR: #673

[ljharb]

Done in #673.