I'd second this - journal entries can pretty much be whatever you want them to be. I'm playing in a game at the moment that has them. You can check out various character journals for R R I here through the character list. As you'll see, some characters use them a lot, some hardly use them at all. The game also allows characters to send each other messages (which are done as though they are journal posts, but just private to one other character).
A good example of lots of different journal styles within the game can be found by looking at my character Kyle's journal, which goes over quite a few entries. His most detailed entry and most 'standard journal entry' type would probably be this one here.