Autodocument enumerated "values" of rule/aspect attributes

Consider the following rule attribute definition:

```
"format": attr.string(
doc = "The format of the output file.",
default = "markdown",
values = ["markdown", "proto"],
),
```

It would be best if Stardoc documented the available values for "format" automatically.

Begin by locating Stardoc's handling of rule and aspect attributes, using the example `format` attribute as the starting case. Done means generated documentation automatically lists the allowed values `markdown` and `proto` for an attribute whose definition supplies a `values` list.

Written by the indexing model from the issue text.