Update isolate docs for new Legacy GCS IAM roles.
Update isolate docs for new Legacy GCS IAM roles.

GCS recently migrated bucket permissions to use IAM roles, and modified
the roles available. The Storage Legacy grouping contains new roles in
place of the previous WRITER role.

R=maruel@chromium.org, mcgreevy@chromium.org, tandrii@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2950333002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py