Prior to this commit, assignment to the `embeds` association happens
_after_ validation callbacks, so it isn't possible to incorporate Rich
Text-related file validation.

For example, consider rejecting `text/plain` files when creating
`Message` records:

```ruby
class Message < ApplicationRecord
  has_rich_text :content

  validate do
    if content.embeds.any? { |attachment| attachment.blob.content_type == "text/plain" }
      errors.add(:content, "Cannot attach text/plain files")
    end
  end
end
```

Without this change, the `content.embeds` association is empty at the
time the `validate` block is evaluated, since that assignment occurs in
a `before_save` callback on the `ActionText::Content` class.

Alongside this change, the commit corrects the documentation on the
`has_many_attached :embeds` line. The returned collection is of
`ActiveStorage::Attachment` records and **not** `ActiveStorage::Blob`
records as documented.