Rambuchan beat me to it. Great advice there, and I agree that giving information on what things do in the pedia is essential, while giving information on what they are/were etc is a very good idea. Here's an extra tip: if you have very long lines of text, the game will insert odd spaces into it when displaying it in-game. So despite what the Firaxis advice says, it's best to split your text into shorter lines rather than have each paragraph in a single line in the text file.