Extend documentation on `ditafileset`

## Description
Extend [`` documentation](https://www.dita-ot.org/dev/topics/plugin-coding-conventions.html) to cover undocumented attributes.
## Possible Solution
The following attributes are supported by the implementation

* `@input = true | false` — `true` for start document, `false` for others; defaults to `false`
* `@inputResource = true | false` — `true` for resources files (`--resource` in CLI), `false` for others; defaults to `false`
* `@conref = true | false` — `true` for documents that use conref, `false` for others; defaults to `false`
* `@processingRole = resource-only | normal` — `resource-only` for documents references as resource only, `normal` for normal references; defaults to any.

## Potential Alternatives
None

## Additional Context
Example uses can be found from e.g. [build_preprocess_template.xml](https://github.com/dita-ot/dita-ot/blob/develop/src/main/plugins/org.dita.base/build_preprocess_template.xml#L158) or [build_preprocess2_template.xml](https://github.com/dita-ot/dita-ot/blob/develop/src/main/plugins/org.dita.base/build_preprocess2_template.xml#L159).