Improve docs of more (potentially re-exported) arti-client types

Most of the structs in arti-client have example code now, to give a clearer idea of how they're used.

Annoyingly, a lot of the types exposed in arti-client are actually re-exports, which makes documentation a bit harder: example code that references other parts of arti-client can't actually be run as a doctest, since the crate it's in is a dependency of arti-client.

We might be able to fix this in future by doing the documentation in arti-client itself, but rustdoc seems to have some weird behaviours there that need to be investigated first (for example, it seems to merge the re-export and original documentation, and also put the re-export documentation on the impl block for some reason).

For now, though, this commit just writes the docs from the point of view of an arti-client consumer, removing notes specific to the crate in which they're defined. It's not ideal, but at least the end user experience is decent.