Fixed docs with return types and throws.

* `ExtendedPdo::connect` throws `\PDOException`.
* `ExtendedPdo::exec` returns affected rows.
* `ExtendedPdoInterface::connect` throws `\PDOException`
  (slash needed, no use).
* `PdoInterface::exec` returns affected rows.
* `PdoInterface::prepare` returns `\PDOStatement`
  (slash needed).
* `PdoInterface::query` returns `\PDOStatement`
  (slash needed, no use).
Merge pull request #67 from ralouphie/develop-2

Fixed docs with return types and throws.