Switch shorthand doc gen over to new approach.

[jamesls]

This PR updates the shorthand documenter to work with the latest changes to the shorthand code (#1461). Specifically:

• It does not use a hardcoded list of "shapes" like the previous approach did.
• It documents the newly supported syntax (nested dictionaries)

It also changes some small things in the way docs are generated:

• The --argname is removed in the shorthand example. This is to make the shorthand syntax consistent with the JSON example syntax.
• The type names are added instead of generic values. Before we'd have Key=value,Foo=value. Now we'll have Key=string,Foo=float.
• The single sentence description has been removed. It's not possible to automatically generate these.

And finally, there's a few areas of improvement I think we'll need to make in the future:

• Right now, shorthand examples are only generated for code of a stack depth of 3 or less. This is because sufficiently complicated structures are not readable in shorthand syntax. We'd need to come up with a way to break these across multiple lines.
• Recursive shapes are documented as (... recursive ...), which is how JSON params are documented. This might be an area for improvement.

cc @kyleknap @mtdowling @rayluo

[kyleknap]

Is SHORTHAND_SHAPES still used?

[jamesls]

Nope, forgot to remove. Will update this.

[kyleknap]

It looks fine. Had a few comments, but I think they would have been caught with pr-check. Did you run it? It would be good to get the tests to pass as well. Probably will need to use OrderedDict

[jamesls]

Feedback should be incorporated. Build is passing.

Looks like there's one test case I missed, I need to add a doc for the changed list type, <value1> .... I'll get a test added for that.

[jamesls]

@kyleknap Ok, I believe everything's been incorporated, including updated tests. Should be good to look at again.

[kyleknap]

Looks good. 🚢