RFR 8189717: Too much documentation of ProcessBuilder.start copied to ProcessBuilder.startPipeline

[Roger Riggs]

Please review a doc change to improve the description of 
ProcessBuilder.startPipeline.
The clarifications are not intended to imply any change in behavior and 
only focus the description
on the steps taken for each command in the pipeline.
(A CSR will be needed also when the wording is finalized).

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-process-8189717/
Issue:
    https://bugs.openjdk.java.net/browse/JDK-8189717

Thanks, Roger