The documentation for the parallel pipeline step is excessively brief. This pull request addresses that by documenting:

• What the criteria for failure and success are
• The (lack of) meaning of the closure return value
• Why some failures are "silent" in that they report no exception (hudson.AbortException)
• How to get results out of the branches if desired
• "Failed in branch" message
• A short practical example
• The return value of the parallel step, and why it should be ignored per JENKINS-26033

See https://github.com/jenkinsci/workflow-cps-plugin/pull/260