[doc fix] Add operation of freeing output SQLDA

Hello

I think it is better to add freeing operation of output SQLDA to the current PostgreSQL documentation.
As far as I can see src/interfaces/ecpg/ecpglib/execute.c,
if a previously existing sqlda is set to output SQLDA, 
then a previously existing sqlda is freed.
But, the new output SQLDA's memory space remain.

ecpg regression test also free output SQLDA's memory space.
The attached patch fixes the documentation.

regards,
--
Kato Sho