Findosg*: Update documentation

- Modules documentation updated and synced.
- Internal comments about headers moved to the modules introduction
  section.
- Added examples sections.
- Used the `<PackageName>_FOUND` variables (CMake 3.3). The uppercased
  `<PACKAGENAME>_FOUND` are also set to the same value.
- Mention of CMake 2.6.3 removed as these version notes are now being
  cleaned in the CMake 4 documentation (CMake docs are at this point
  considering the 2.8.12.2 as the initial state upon which new and
  changed items are documented through versions).
- Added notes that these modules are not intended anymore to be used
  directly except for advanced case scenarios. The FindOpenSceneGraph
  module should be used.
- Fixed osgQt description in FindOpenSceneGraph and synced it a bit.