Cubemap documentation issues

[Shimmen]

There are several functions in the API that accepts both Cubemaps and Textures, but in most places only Texture is documented as a valid parameter type. Examples are DrawCall.texture, Framebuffer.colorTarget and Framebuffer.depthTarget.

There are also some missing details around creating cubemaps, specifically for cubemaps without any input textures.

[tsherif]

Fixed by #95