Document experimental GDK settings

What does this Merge Request do?

This MR documents the current GDK experimental settings. The gdk.experimental.ruby_services setting is not documented currently because it will cause issues and will be removed in a separate MR and included as part of #904.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.