[JENKINS-17351] won't fix - clarified help text

In [JENKINS-17351](https://issues.jenkins-ci.org/browse/JENKINS-17351), the issue reporter tried to archive files from outside the workspace.

To my understanding, this is not possible, see my comments in the issue. I'd like to close the issue with resolution _won't fix_. 

This pull request is an enhancement of the English and German help message that clarifies the subject.

Can someone please verify that my assumptions about artifact resoltution are correct?