Ported documentation fixes from mysql-simple

Taken from bos/mysql-simple@620a6390e93ce22b247b3e599305cd1a08dac4e3

Turns out that issue #108 was fixed in mysql-simple before postgresql-simple's
first release,  but before I started examining mysql-simple.   So by some
mechanism,  I started work on postgresql-simple from a slightly out-of-date
version of mysql-simple.