Document sourceMigration limitation with multiple map keys

I think it would be helpful if migrate_example_baseball include a comment regarding the use of multiple map keys and how that prevents sourceMigration() from working as expected. Attached is a patch that adds the following comment just before $this->map = new MigrateSQLMap($this->machineName in migrate_example_baseball.migrate.inc:

    /**
     * Note that using more than one key in a map object (as in this example)
     * will prevent sourceMigration() from working as expected when mapping
     * destination fields.
     * See https://www.drupal.org/node/2217909 for a discussion of this issue.
     */