FindMPEG, FindMPEG2: Update documentation

- Modules documentation updated and synced.
- Added examples section.
- FindMPEG and FindMPEG2 modules behave very much the same except that
  FindMPEG2 also adds the SDL library as a dependency. Added notes
  regarding this and recommended to use the FindMPEG2 as it also links
  in the SDL library at the time of writing.