Document how to emit attribute defaults (and other customizations) via the SDK

Assume the following schema segment:

```go
"quantity": {
Default: 30,
Description: "The quantity of this resource.",
Optional: true,
Type: schema.TypeInt,
}
```

When documentation is generated, fields are emitted with their type and a requirement "indicator":

> **quantity** (Number, Optional) The quantity of this resource.

A default value should be emitted to the generated Markdown if specified in the underlying schema:

> **quantity** (Number, Optional, Default: 30) The quantity of this resource.