Improve sample template

## Description
Make it clear how the following sample template correspondence with DITA.

[https://github.com/dita-ot/docs/blob/develop/topics/plugin-rewrite-rules.dita](https://github.com/dita-ot/docs/blob/develop/topics/plugin-rewrite-rules.dita)

- What is "file" element?
- What is "@result" attribute?
- Does above corresponds to actual DITA element & attributes? Or they are only the example?

## Possible Solution
Add description to above template code.

## Potential Alternatives
None

## Additional Context

> To dynamically adjust the names and locations of output files in the map-first pre-processing routine (preprocess2)

It is also unclear that this function guarantee the xref/@href rewrite or reltable references.