Fix comment on processes being kept over a restart

Fix comment on processes being kept over a restart

All child processes except the syslogger are killed on a restart. The
archiver might be already running though, if it was started during
recovery.

The split in the comments between "other special children" and the
first group of "background tasks" seemed really arbitrary, so I just
merged them all into one group.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/8f2118b9-79e3-4af7-b2c9-bd5818193ca4@iki.fi