FindFLTK, FindFLTK2: Update documentation

- Modules documentation updated and syced.
- Added examples section.
- Mentioned FLTK upstream package configuration file that is also taken
  into account when calling the find_package(FLTK).
- Briefly mentioned the fltk_wrap_ui() command.
- Added note for FindFLTK2 that it is for the obsolete 2.x branch that
  wasn't released and FindFLTK should be used instead.

Issue: #15728