RenderToCubemap(cubemap: Cubemap) docs and error message could be improved.

That function actually reads back an image from the GPU for each face and the uploads the cubemap to the GPU. It is slow and should only be used in an Editor tool or at the start of a level. That should be added to the docs.

When used on a Cubemap without the Readable flag set an error massage saying "GetRawImageData()" is spit out. Not ideal.