Add GMA feature flag name to docs

What does this MR do?

Previously the documentation for group-managed accounts did not mention the name of the feature flag its hidden behind. This adds a small section explaining the feature flag name.

A note has been added to remove this documentation in the issue to eliminate this feature flag.

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