Doc: fix return example as @example only allows one line

[lulalala]

The doc does not display example of return option properly in https://www.rubydoc.info/gems/elasticsearch-model/Elasticsearch/Model/Importing/ClassMethods:import

This is because yard doc only allows @example title to be single line, but here we made it multi-line.

I've also updated the @return tag to indicate it can be changed.

Lastly, "Update the batch before yielding it" example does not render the last code line, because the indentation is inconsistent. That's fixed too.

[estolfo]

Thanks @lulalala !