add some docs for the cache dir (#10568)
add some docs for the cache dir (#10568)


hashicorp/packer,rnaveiras/packer,mitchellh/packer,pecigonzalo/packer,rnaveiras/packer,pecigonzalo/packer,ricardclau/packer,rnaveiras/packer,ricardclau/packer,pecigonzalo/packer,rnaveiras/packer,pecigonzalo/packer,mitchellh/packer,hashicorp/packer,mitchellh/packer,rnaveiras/packer,hashicorp/packer,hashicorp/packer,ricardclau/packer,mitchellh/packer,ricardclau/packer,rnaveiras/packer,ricardclau/packer,ricardclau/packer,mitchellh/packer