Updates doc for Protected Environments

What does this MR do?

Updates documentation for Protected Environments to explicitly state that the feature is under a feature flag

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7535

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides
• [-] EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?