max_by/min_by specification disagrees with the examples

The max_by and min_by specification declares that the 2nd argument is an expression that returns either a number or a string.

`expression->number|expression->string`

However, they both have an example that specifically contradicts this showing that strings are not allowed. Which is it?

|Expression | Result|
|-- | --|
|max_by(people, &age_str) | |
|min_by(people, &age_str) | |