In Editor/Data/Documentation/en/Manual/class-Cubemap.html, this paragraph is outdated:

"You can also use the Generate Cubemap option on the Texture Importer to produce a cubemap automatically from a single image. The importer will wrap the image around and join the edges but you will need to start with an image designed for this purpose to get the best results."

I think it should just say smth like: you can also just choose Texture Type Cubemap to generate a Cubemap from a single image designed for this purpose.