It depends on the content you want to talk about. A general warning won’t help to avoid trigger, stress or anxiety. Better to write your text, read over it, put it in spoilers and prepend content notes (CN) about the included points. These can be various traumatizing aspects and you can be really detailed as people have various stress symptoms. Food, harrassment, bullying, sexism, death - everything you talk about you can warn your readers with content notes.