feature request: add description field to job

### Proposal
I would like to propose a "description" field to the job specification to allow operations people adding custom stuff to it.
The field would be string and will be totally ignored. Updating the field would be a non-destructive update and will be invisible to the task and will have no effect on anything.
Also, it could be displayed below the job name on the web UI job view and as a mouse hover above the job on jobs view.

### Use-cases
The use case it to record some metadata with the job, like who deployed it, or from where, who is responsible for it, and to be possible to change this metadata _without_ restarting the job.

### Attempted Solutions
Updating meta of the job counts as a destructive update.
It's possible to add another datacenter and use it as a comment.
Service meta can be used, if there is no templating depending on it.
Another empty but valid group with count=0 can be used, where you can store anything in meta of this group.

Thanks!