Document how the special keys in SystemData.cpp are used

Special keys in system key space (\xff) defined in SystemData.cpp are used for managing the DB (e.g., disable data distributor) or storing metadata for various DB functionalities.

There is no documentation about them for now.

It will be helpful for both developers and users by document each key or key range:
1) What functionality or feature it is used for?
2) What format it uses or what data does it carry?
3) How to use it from a DBA perspective?

Start by reading SystemData.cpp and tracing each special key or key range in the surrounding database-management and metadata code. Document the functionality, stored format or data, and DBA-facing usage for every identified key or range; done means developers and users can find this information in project documentation.

Written by the indexing model from the issue text.