Allow people to attach a description to a capability

[Michael-F-Bryan]

Something @Ge-te pointed out is it'd be really nice if capabilities had a description field. That way you can give a better explanation of what is expected from a particular capability (e.g. the style image in style_transfer is an image whose style is applied to something else).

We could achieve this by either adding an explicit description field at a capability's top level or establishing a convention that when a capability's args field contains a description string we'll use that to describe the capability.