HDFS-12833. Distcp : Update the usage of delete option for dependency with update and overwrite option. Contributed by usharani.

(cherry picked from commit 8d69b7f2d41e6a76c8c706ead21b1abe86592881)

hadoop-tools/hadoop-distcp/src/main/java/org/apache/hadoop/tools/DistCpOptionSwitch.java

@@ -73,7 +73,8 @@ public enum DistCpOptionSwitch {
    */
   DELETE_MISSING(DistCpConstants.CONF_LABEL_DELETE_MISSING,
       new Option("delete", false, "Delete from target, " +
-          "files missing in source")),
+          "files missing in source. Delete is applicable only"
+          + " with update or overwrite options")),
 
   /**
    * Configuration file to use with hftps:// for securely copying

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

@@ -230,7 +230,7 @@ Flag              | Description                          | Notes
 `-filters` | The path to a file containing a list of pattern strings, one string per line, such that paths matching the pattern will be excluded from the copy. | Support regular expressions specified by java.util.regex.Pattern.
 `-filelimit <n>` | Limit the total number of files to be <= n | **Deprecated!** Ignored in the new DistCp.
 `-sizelimit <n>` | Limit the total size to be <= n bytes | **Deprecated!** Ignored in the new DistCp.
-`-delete` | Delete the files existing in the dst but not in src | The deletion is done by FS Shell. So the trash will be used, if it is enable.
+`-delete` | Delete the files existing in the dst but not in src | The deletion is done by FS Shell. So the trash will be used, if it is enable. Delete is applicable only with update or overwrite options.
 `-strategy {dynamic|uniformsize}` | Choose the copy-strategy to be used in DistCp. | By default, uniformsize is used. (i.e. Maps are balanced on the total size of files copied by each map. Similar to legacy.) If "dynamic" is specified, `DynamicInputFormat` is used instead. (This is described in the Architecture section, under InputFormats.)
 `-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.