JFR: Improve javadoc for @Name

The documentation of the jdk.jfr.Name annotation should contain more information to guide users. In particular,

- The event in the example should not be called "MyEvent". Using the "Event"-suffix is not recommended.

- We should provide information of what constitutes a good name, reverse domain notation etc.

See:

https://github.com/flight-recorder/samples/blob/master/Sample_13_Name.java