Revise examples and description for traits

[JJ]

Problem or new feature

Examples for traits seem to imply traits are called at compile time, as it clearly says in the description, compile-time hooks, which are run exactly once when the declaration takes place. This is not really clarified by the examples, which call the traited code exactly once. In reality, hooks are only called at compile time.

Suggestions

The examples are probably OK, if a few changes in variable names and how they behave is changed. See also #2714