Confusion in the default builder

The default builder claims to support a wide variety of platforms for multi-platform images, but is actually incapable of building anything beyond the host architecture.

$ docker buildx inspect default
Name:   default
Driver: docker

Nodes:
Name:      default
Endpoint:  default
Status:    running
Platforms: linux/arm64, linux/amd64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6

Lies! Try to use the default builder for things besides the host architecture results in error.

Ideally, multi-platform builds would actually be enabled automatically for the default builder. With automatic loading of host-supported platform images into the main image cache.

If, for some reason we do not want to enable multi-platform builds out of the box on the default builder, then at least update the list of platforms there to say just linux/<host architecture>, for clarity.