Clarification to the "prohibit outer forks" docs

What does this MR do?

This clarifies the documentation for the new "Prohibit outer forks" feature for group-managed accounts. Specifically, it notes the location of the configuration option in response to a question in the feature issue.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content