Merge "Make docs clear that you shouldn't change an existing onUpgrade step." into udc-dev

     * This method executes within a transaction.  If an exception is thrown, all changes

     * will automatically be rolled back.

     * </p>

     * <p>

     * <em>Important:</em> You should NOT modify an existing migration step from version X to X+1

     * once a build has been released containing that migration step.  If a migration step has an

     * error and it runs on a device, the step will NOT re-run itself in the future if a fix is made

     * to the migration step.</p>

     * <p>For example, suppose a migration step renames a database column from {@code foo} to

     * {@code bar} when the name should have been {@code baz}.  If that migration step is released

     * in a build and runs on a user's device, the column will be renamed to {@code bar}.  If the

     * developer subsequently edits this same migration step to change the name to {@code baz} as

     * intended, the user devices which have already run this step will still have the name

     * {@code bar}.  Instead, a NEW migration step should be created to correct the error and rename

     * {@code bar} to {@code baz}, ensuring the error is corrected on devices which have already run

     * the migration step with the error.</p>

     *

     * @param db The database.

     * @param oldVersion The old database version.