GitDownloadCommand#getRepoCommand: Add Javadoc to prevent 'unused' warnings

Change-Id: Ib4ab2165755c9889ed01dc57318a63a7fb682b86
diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/command/GitDownloadCommand.java b/src/main/java/com/googlesource/gerrit/plugins/download/command/GitDownloadCommand.java
index 62f01a3..e47a31a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/command/GitDownloadCommand.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/command/GitDownloadCommand.java
@@ -136,8 +136,12 @@
 
   abstract String getCommand(String url, String ref);
 
-  // Most commands don't support this, so default it to nothing.
+  /**
+   * @param url The project URL this change is for.
+   * @param id The CL/PS numbers.
+   */
   String getRepoCommand(String url, String id) {
+    // Most commands don't support this, so default it to nothing.
     return null;
   }
 }