Doc improvements for sandbox creation (#1032)
Doc improvements for sandbox creation (#1032)

- Make it clearer how to experiment with private sandbox configuration
  value changes (make repo names greppable)
- Clarify warnings about hyphens and underscores

https://github.com/edx/jenkins-job-dsl/pull/765 is when the warning about
hyphens was originally introduced:

> If I remember correctly, the issue may have arisen when trying to
> access `portal-designer` on a Sandbox.
edx/jenkins-job-dsl