Document CloneCommand.setURI(null)

call() throws InvalidRemoteException if uri == null, so there should
never be reason to leave the URI set to null.  Document this.

Change-Id: I7f2cdbe8042d99cf8a3c1a8c4c2dcb58c5b8c305
Signed-off-by: Jonathan Nieder <jrn@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/CloneCommand.java

@@ -331,7 +331,8 @@ private RevCommit parseCommit(final Repository clonedRepo, final Ref ref)
 
 	/**
 	 * @param uri
-	 *            the uri to clone from
+	 *            the URI to clone from, or {@code null} to unset the URI.
+	 *            The URI must be set before {@link #call} is called.
 	 * @return this instance
 	 */
 	public CloneCommand setURI(String uri) {