Update tiered support documentation to clarify feature support levels and enhance readability

- Revised descriptions for Tier A to specify that features have been used in products.
- Reformatted notes on testing, maintenance, and support definitions for better clarity.
- Added a new section for privileged features with detailed explanations.
- Updated tables for Tier A, Tier B, and Tier C to include additional targets and compilation flags.
- Removed outdated comments and added TODOs for future enhancements.

Author: lum1n0us