What is the relationship between documentary photography and thematic photography?

You can tell the difference from the name.

Documentary, of course, reflects the true face of the facts, and it is not allowed to modify or change the nature of the picture later. News photography basically falls into this category.

Special topics are hard to say. It is too wide. For example, taking a special topic in Africa, taking a special topic in kindergarten or taking a special topic by plane can all be called thematic photography.