Fix a bug in AbstractSyncableContentProvider.java and modify comments.

    /**

     * <p>

     * Call mOpenHelper.getWritableDatabase() and mDb.beginTransaction().

     * {@link #endTransaction} MUST be called after calling this method.

     * Those methods should be used like this:

     * Start batch transaction. {@link #endTransaction} MUST be called after 

     * calling this method. Those methods should be used like this:

     * </p>

     *

     * <pre class="prettyprint">

     * boolean successful = false;

     * if (!beginTransaction()) {

     *     return;

     * }

     * beginBatch()

     * try {

     *     // Do something related to mDb

     *     successful = true;

     *     return ret;

     * } finally {

     *     endTransaction(successful);

     *     endBatch(successful);

     * }

     * </pre>

     *

     * @hide This method should be used only when {@link #applyBatch} is not enough and must be

     * used with {@link #endTransaction}.

     * used with {@link #endBatch}.

     * e.g. If returned value has to be used during one transaction, this method might be useful.

     */

    public final void beginBatch() {

    /**

     * <p>

     * Call mDb.endTransaction(). If successful is true, try to call

     * Finish batch transaction. If "successful" is true, try to call

     * mDb.setTransactionSuccessful() before calling mDb.endTransaction().

     * This method MUST be used with {@link #beginBatch()}.

     * </p>

                getContext().getContentResolver().notifyChange(url, null /* observer */,

                        changeRequiresLocalSync(url));

            }

            mDb.endTransaction();

        }

    }