Add a `description` field

[liamhuber]

This was a request from a conversation with @XzzX, for presenting intent in a node store situation.
Where a reference python function is available, this can just be scraped from there (as @XzzX does in the current node store implementation), but workflows are formally allowed to be independent of a reference python function and there's no reason to disallow someone manually (or otherwise) filling the description field.

I could see putting this right on the base node model, or adding it specifically to WorkflowNode and AtomicNode and omitting it on the flow controls. Right now I guess I lean towards making it universal and defaulting to empty.