FindPackageMessage: Update documentation

- Documentation updated.
- This module is internally already used by the
  FindPackageHandleStandardArgs module and can conflict when used again
  in the find module due to the same <PackageName> used, or when QUIET
  option is used. For this reason, the example is changed and explained
  at the bottom to use find_package_handle_standard_args().
- Lower-case style function used in FindPackageHandleStandardArgs.