Clarify how to vary `out` filename based on configuration

Custom output requires that the `out` attribute be configurable. E.g.

```
out = select({
"//:xml": "ocaml_archive.xml",
"//conditions:default": "ocaml_archive.md"
}),
```

No file or test is named. Start by finding the documentation for the configurable `out` attribute and the `select` example; done means the docs clearly explain how to vary the output filename by configuration and include the shown XML/Markdown case.

Written by the indexing model from the issue text.