Document automatically_translated file format support

When updating units from files, `supports_explanation` flag on the file_format_cls is used to detect whether a explanation change is metadata or data change.

Currently automatically translated, fuzzy and approved states are handled in format with fallback from a database, it would also make sense to make it a feature flag on the format (that would be useful for documentation purposes as well) and handle it on the model side.

_Originally posted by @nijel in https://github.com/WeblateOrg/weblate/pull/17426#discussion_r2637718685_