* Improve ActionText::FixtureSet documentation
Support for Action Text attachments in fixtures was added by [76b33aa][] and
released as part of [6.1.1][], but has not yet been documented.
This commit documents the `ActionText::FixtureSet` for the API
documentation, and mentions it in the Rails Guides pages.
[76b33aa]: 76b33aa3d1
[6.1.1]: https://github.com/rails/rails/releases/tag/v6.1.1
* Fix indention of comments
Co-authored-by: David Heinemeier Hansson <david@loudthinking.com>