Improve etnservice function documentation and examples

[PietrH]

Currently the function documentation (roxygen) within etnservice is the same as it was on etn, except for the adapted parameter credentials which replaces connection. The examples haven't been changed at all (so will not work).

This documentation is currently not user facing, as etn uses it's own sql helper and roxygen documentation (see inbo/etn#280). Meaning that the database interfacing code needs to be maintained in two places. However, we are considering reducing this to one place by making etnservice a dependency of etn and using/evaluating/parsing the code from etnservice for use in the database interfacing functions of etn.

In short, either we should update the documentation in etnservice to be more userfriendly for future collaborators, or we should update it to be correct and client facing once more, so it can be inherited by etn.