That's a mistake, I'll fix it in the docs. Sorry about that.
Looks like it's not fully covered by guides except that short example with renaming keys section in mapper guide.
In your examples you show an sql relation so I assumed you're using rom-sql, this means you can use standard query dsl to cherry-pick which attributes a given relation returns. When using repositories you can define relation views explicitly and it's described in the repository guide.
In most of the cases you can leverage relation API to define lower-level data structs that relations return, this is really all you need in majority of the cases and it is faster since it's using native db features. On top of it repositories give you a simple interface to compose relations together that will produce aggregates, something that would require manual mapping configuration if you didn't use repositories.
For now the only piece of docs about repository is the repository guide I linked above.
Ah, so you're not using SQL? I'm not sure what your use case is exactly so I can't provide more information.
Sorry to "hear" that. What are you going to use instead, if I may ask?