docs(jenkins): Update Jenkins documentation

[mstock]

Current versions of the karma-junit-reporter and the given sample configuration store the report below directories named after the browsers the tests were run in, which must be reflected in the Jenkins configuration.

We recently ran into this issue after we updated to karma-junit-reporter 0.3.7, and noticed that the documentation in the Karma documentation did not reflect these changes (but the original karma-junit-reporter documentation does). So I'd suggest the documentation changes in this PR, but feel free to adapt/change them, especially since I'm not a native speaker.

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[mstock]

@googlebot I signed it!

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[dignifiedquire]

Thanks