docs: template signal change_mode not compatible with env
docs: template signal change_mode not compatible with env

Only `change_mode = "restart"` will result in the template environment
variables being updated in the task. Clarify the behavior of the unsupported
options.

hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad