FeatureSummary: Enhance documentation

This fixes typos, syncs docs code style a bit and refactors functions
descriptions.

- Initial example improved and synced with the current
  `feature_summary()` output
- The global properties are listed with indentation
- Added reference links where it makes sense to better understand the
  terminology
- CMake versions added for all properties (they got introduced in CMake
  3.8)
- All "macro" words renamed to "function"
- "Legacy Macros" renamed to "Deprecated Functions"