Add a few more non-extendable interface

Signed-off-by: Carl Schwan <carl@carlschwan.eu>

lib/public/DB/IPreparedStatement.php

 use PDO;
 
 /**
+ * This interface allows you to prepare a database query.
+ *
+ * This interface must not be implemented in your application but
+ * instead obtained from IDBConnection::prepare.
+ *
+ * ```php
+ * $prepare = $this->db->prepare($query->getSql());
+ * ```
+ *
  * @since 21.0.0
  */
 interface IPreparedStatement {

lib/public/DB/IResult.php

 use PDO;
 
 /**
+ * This interface represents the result of a database query.
+ *
+ * Usage:
+ *
+ * ```php
+ * $qb = $this->db->getQueryBuilder();
+ * $qb->select(...);
+ * $result = $query->executeQuery();
+ * ```
+ *
+ * This interface must not be implemented in your application.
+ *
  * @since 21.0.0
  */
 interface IResult {

lib/public/DB/ISchemaWrapper.php

 use Doctrine\DBAL\Platforms\AbstractPlatform;
 
 /**
- * Interface ISchemaWrapper
+ * This interface allows to get information about the database schema.
+ * This is particularly helpful for database migration scripts.
+ *
+ * This interface must not be implemented in your application but
+ * instead can be obtained in your migration scripts with the
+ * `$schemaClosure` Closure.
  *
  * @since 13.0.0
  */