On this page:
https://docs.mongodb.com/manual/core/capped-collections/#automatically-remove-data-after-a-specified-period-of-time

This should say "As an alternative to capped collections, you could use TTL indexes..."

As written, it says for "additional flexibility", which implies you can add it to capped collections, because that is the topic on this page. Then the last line of the section says it can't be used for capped collections, which is confusing