JavaDoc fixes for Java/RESTEasy client

[delenius]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

I added/fixed JavaDoc comments in the mustache templates for the Java/RESTEasy client, so that it no longer generates errors and warnings when running javadoc on the generated code.
I also modified the pom template so that it creates the javadoc automatically, to be consistent with the other clients.

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[delenius]

Thanks for the heads up, forgot about that. Will fix momentarily...

[wing328]

The change looks good to me. I'll merge it once the CI tests pass.

[jmini]

CircleCI fail is not related to this change.

Thank you a lot for this change.