FindCxxTest: Update documentation

- Synced module documentation with other similar find modules.
- Extended example with imported target.
- Removed mentioning CMake 2.x in versionadded RST directives as these
  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 - at least for
  modules).
- Added separate "Deprecated Variables" section.
- Changed the deprecation version number to 2.8.3 as it is for CMake
  version and not the module internal version 1.3.
- Added separate "Cache Variables" section.
- Used lowercase style for the cxxtest_add_test() command.
1 file changed