mesos.proto comment falsely claims that non-persistent volumes are supported

According to mesos.proto,

// NOTE: If 'volume' is set but 'persistence' is not set, the
// volume will be automatically garbage collected after
// task/executor terminates. Currently, if 'persistence' is set,
// 'volume' must be set.

However, when actually trying to set 'volume' but not 'persistence', one gets a TASK_ERROR with the message "Task uses invalid resources: Invalid DiskInfo: Non-persistent volume not supported".

I've run into this on 1.5.2, but I can see that validation code is still present on master so I've tagged this bug for 1.7.0 as well. I will submit a patch to change the documentation in mesos.proto.