Added "Memory observability" to javadoc.

 *     <li>The task can be executed only once (an exception will be thrown if

 *     a second execution is attempted.)</li>

 * </ul>

 *

 * <h2>Memory observability</h2>

 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following

 * operations are safe without explicit synchronizations.</p>

 * <ul>

 *     <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them

 *     in {@link #doInBackground}.

 *     <li>Set member fields in {@link #doInBackground}, and refer to them in

 *     {@link #onProgressUpdate} and {@link #onPostExecute}.

 * </ul>

 */

public abstract class AsyncTask<Params, Progress, Result> {

    private static final String LOG_TAG = "AsyncTask";