Update "acl ch" docs to explicitly state that service accounts are considered to be users.
Update "acl ch" docs to explicitly state that service accounts are considered to be users.

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