HDFS-10298. Document the usage of distcp -diff option. Contributed by Takashi Ohnishi.

hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm

@@ -232,7 +232,7 @@ Flag              | Description                          | Notes
 `-bandwidth` | Specify bandwidth per map, in MB/second. | Each map will be restricted to consume only the specified bandwidth. This is not always exact. The map throttles back its bandwidth consumption during a copy, such that the **net** bandwidth used tends towards the specified value.
 `-atomic {-tmp <tmp_dir>}` | Specify atomic commit, with optional tmp directory. | `-atomic` instructs DistCp to copy the source data to a temporary target location, and then move the temporary target to the final-location atomically. Data will either be available at final target in a complete and consistent form, or not at all. Optionally, `-tmp` may be used to specify the location of the tmp-target. If not specified, a default is chosen. **Note:** tmp_dir must be on the final target cluster.
 `-async` | Run DistCp asynchronously. Quits as soon as the Hadoop Job is launched. | The Hadoop Job-id is logged, for tracking.
-`-diff` | Use snapshot diff report to identify the difference between source and target. |
+`-diff <fromSnapshot> <toSnapshot>` | Use snapshot diff report between given two snapshots to identify the difference between source and target. | This option is valid only with `-update` option and the following conditions should be satisfied. 1. Both the source and target FileSystem must be DistributedFileSystem. 2. Two snapshots (e.g., s1 and s2) have been created on the source FS. The diff between these two snapshots will be copied to the target FS. 3. The target has the same snapshot s1. No changes have been made on the target since s1. All the files/directories in the target are the same with source.s1. |
 `-numListstatusThreads` | Number of threads to use for building file listing | At most 40 threads.
 `-skipcrccheck` | Whether to skip CRC checks between source and target paths. |