Confusing examples in 4.5

[Chapter 4.5](https://github.com/airbnb/javascript/blob/master/README.md#arrays--callback-return) contains some confusing examples.

1. `map` examples have not corresponding bad examples. What bad cases they advise against?
2. `reduce` bad example is just wrong, it throws `TypeError: Cannot read property 'concat' of undefined`. `reduce` callback can't be used without `return` statement.
3. `filter` examples seem to be just a case of something like ['no-else-return'](http://eslint.org/docs/rules/no-else-return) ESLint rule.

Open Chapter 4.5 in README.md and review the map, reduce, and filter examples described in the issue. Check the reduce example's behavior and compare the filter guidance with the linked no-else-return rule. Done means the examples are correct and each listed concern is clearly addressed.

Written by the indexing model from the issue text.