Improve some comments in pg_checksums about the needed clean shutdown

It was not clear from the code why it is necessary.  And we need a clean
shutdown to avoid random checksum failures caused by torn pages.

Author: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoDum5MbAb7F=pa9dOD1W2tukuDMPzWT7NjZceNoWB_6Qw@mail.gmail.com
