Shared journals are undocumented because the process of creating one is a bit cumbersome, and because Brad has left open the possibility that they will be phased out altogether.
Shared journals allow entries to be posted from other accounts, just like communities. So, why wouldn't you just use a community? Normally, you would. But communities have a couple of features which may not always be quite what you want.
First, communities have instructions on the Community Info page, which explain how someone can either join or inquire about doing so. If you never intend to let anyone else join, you might not want those instructions there. Converting the community to a shared journal replaces the Community Info page with an ordinary User Info page, so the instructions about joining disappear. That's useful to know, if you want more than one journal for personal use but you don't want to bother logging in under multiple usernames.
Second, entries in community journals are always arranged according to when they were actually posted. If you want to let other accounts post to a journal, but you still want backdated or postdated entries to be arranged according to dates entered manually, what you need is a shared journal.
However, a shared journal cannot have open membership like many communities do. The maintainer must personally grant permission to post entries and view friends-only entries.
How to create and maintain one
Before you can turn a journal into a shared journal, you must already have turned it into a community, with another account as its maintainer. While it's a community, you'll probably want to take the opportunity to join it, which allows you to post to the journal and see its friends-only entries. Then log in to the community account (not the maintainer's account!), and go to the command console. Execute the command
… replacing comm_name by the community's actual username. Now you have a shared journal.
change_journal_type comm_name shared
To change who can post to the shared journal, log in to the maintainer's account and use the console's shared command. It works the same way for both shared journals and communities.
To change who can see friends-only posts in a shared journal, you must log in to the shared-journal account and edit its Friends list.
To convert a shared journal back to some other type, you must return to the Create Community page. After the shared journal has been converted back to a community, you can again use the console's change_journal_type command.