FindOpenThreads: Update documentation

- Added examples section.
- Used OpenThreads_FOUND variable. The uppercased OPENTHREADS_FOUND is
  also set to the same value as of CMake 3.3.
- Module documentation synced with other similar find modules.
- Mention of CMake 2.8.10 removed as these are now being cleaned in the
  CMake 4 documentation (CMake docs are at this point considering the
  3.0 as the initial state upon which new and changed items are
  documented through versions).
- Refactored the hints variables section.
- Added info about release and debug variants.
- Refactored internal comments into more readable documentation
  paragraph.  Most of the issues described in the internal comment have
  been resolved in CMake evolution by now - CMake now offers _ROOT
  variables and CMAKE_PREFIX_PATH variable which works by finding that
  paths with precedence over default standard locations.