Improve paramconverter docs
merged branch mdavis1982/improve_paramconverter_docs (PR #240)

This PR was squashed before being merged into the 2.2 branch (closes #240).

Discussion
----------

Improve paramconverter docs

Added a tip to the `@ParamConverter` documentation about passing values other than a numeric ID to a repository method.

```
| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (added tip)
| Applies to    | all
| Fixed tickets | none
```

Commits
-------

a9da994 Improve paramconverter docs