AQP4 and neuromyelitis optica: Comorbidities occur in aquaporin‐4 antibody‐positive neuromyelitis optica spectrum disorder (AQP4‐NMOSD), myelin oligodendrocyte glycoprotein antibody‐associated disease (MOGAD), and double seronegative NMOSD (DN‐NMOSD), potentially contributing to a less favorable disease course.