Added explicit warning about using streaming uploads/downloads for large amounts of data to cp help
Added explicit warning about using streaming uploads/downloads for large amounts of data to cp help

GoogleCloudPlatform/gsutil,ttiurani/gsutil,BrandonY/gsutil,chriskuehl/gsutil-debian,craigcitro/gsutil,dtjackson/gsutil,GoogleCloudPlatform/gsutil,dimfeld/gsutil,fishjord/gsutil,mattdr/gsutil