For my comic, I focus the most on details about the universe that are the most relevant to the plot, and establishing individual characters, and the character classes and factions they're a part of. Before I got to writing notes for the factions, I already had notes for individual characters. For each character class, I needed to establish their strengths and weaknesses, and I also needed to establish what the rules of magic and time travel are, including which rules can be broken, and which rules never can.
Because the plot of my comic involves different factions, I also need to go into detail about their histories, their alliances and rivalries with each other, their ideologies, and political differences. Taking place in a fictional country, I also focus on showing the setting, showing the climate, the architecture, history and culture. I wouldn't need to go into too much detail about every landmark or city, and just include the places that are relevant to the plot in the comic.
In my notes, I've written more about the lore than I might need to include in the comic itself, but I feel like it's better to write more in my notes than I needed, than to not have enough. The extra information could always be used in supplementary materials.