The search option in the documentation is flawed. It took me a very long time to find the manual page that explains the different Texture Importer. Turned out the page is called 2D Textures: http://docs.unity3d.com/Manual/class-TextureImporter.html and apparently it can only be found by searching for the term "2D Textures". I was searching for "Texture importer", "Texture importing", "Texture import options", etc. and couldn't find that page. The words "Texture importer" appear on the "2D textures" manual page, so the search should return that as a result.