Generate documentation for custom commands with JSON input.

Generates documentation about the JSON shorthand and long form input for
custom arguments that have a schema attribute set. The output looks
like:

```
--foo (structure) Help text goes here

Shorthand Syntax:

  --foo arg1=value,arg2=value

JSON Syntax:

  {
    "arg1": "string",
    "arg2": integer
  }
```