FindBZip2: Update documentation

- Added examples section.
- Descriptions reworded and synced a bit with other find modules.
- BZIP2_NEED_PREFIX variable moved to cache variables
- BZip2_FOUND variable described instead of BZIP2_FOUND to be synced
  with other find modules and current naming conventions. Both of these
  variables are set to the same value in this case.
1 file changed