Clarify service parameters usages

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | IRC

On the IRC channel, there was some confusing about parameters. The dot
notation looked like parameters are one big namespaced tree, with each
group a different node.

This PR adds a caution, so people really know it's just a convention and
parameters are just key-value elements.