prepared transactions improvements

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.3/static/sql-prepare-transaction.html
Description:

Under PREPARE TRANSACTION it says that transaction_id must be unique, it
doesn't say what happens when it is not unique.

It might be useful to add a note mentioning max_prepared_transactions needs
to be configured before prepared transactions are enabled, although there's
already a note saying to leave it at 0 if prepared transactions are not
required.

Under COMMIT PREPARED and ROLLBACK PREPARED it should also say what happens
if transaction_id does not exist.

Thanks,
Jim