Then you should not put information like this in the manual:
4.2.2 Automatically Deleting Old Spam Messages
Mail has a feature, accessible from the Mailbox Behaviors (or Special Mailboxes) tab of the Accounts preferences, that can automatically delete old spam messages from the Junk mailbox. You can use this feature if you tell SpamSieve to put spam messages in the Junk mailbox instead of the Spam mailbox.
(Note: If you do this, be careful not to use the Junk and Not Junk buttons that will appear in Mail when the Junk mailbox is active; you should always use the SpamSieve - Train as Good and SpamSieve - Train as Spam commands instead. You can use the Customize Toolbar… command in Mail’s View menu to remove the Junk button from the toolbar. Also, if you select a message that SpamSieve has classified as spam, Mail will show a banner saying that you marked it as junk. Ignore this.)
To tell SpamSieve to use the Junk mailbox, open Mail’s Preferences window and click on Junk Mail. Enable Mail’s junk mail filtering in Automatic mode, then click Advanced… and edit the rule such that the conditions don’t match any messages. For example, use these two conditions:
Message is addressed to my full name
Message is not addressed to my full name
Click OK to close the sheet. Do not make any further changes to the Junk Mail preferences. It should still be in Automatic mode, and you can ignore the rest of the window.
Go to the Rules section of Mail’s preferences and change the SpamSieve rule to move the messages to the Junk mailbox instead of the Spam mailbox. Finally, choose SpamSieve - Change Settings from Mail’s Message menu and, when prompted, say that the name of your spam mailbox is Junk.