Complete the document for FileDistribution processor in OfflineImageViewer

The document for FileDistribution processor seems not completed. These are two points:

• The documentation for new options maxSize, step and format were not added in hdfs oiv_legacy. However, these options are making sense.
• After merging HDFS-10778, it introduced a new option option to format the output result. But the introduction for FileDistribution was not updated.