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

src/bin/pg_checksums/pg_checksums.c

@@ -475,7 +475,7 @@ main(int argc, char *argv[])
 		exit(1);
 	}
 
-	/* Check if cluster is running */
+	/* Read the control file and check compatibility */
 	ControlFile = get_controlfile(DataDir, &crc_ok);
 	if (!crc_ok)
 	{
@@ -497,6 +497,11 @@ main(int argc, char *argv[])
 		exit(1);
 	}
 
+	/*
+	 * Check if cluster is running.  A clean shutdown is required to avoid
+	 * random checksum failures caused by torn pages.  Note that this doesn't
+	 * guard against someone starting the cluster concurrently.
+	 */
 	if (ControlFile->state != DB_SHUTDOWNED &&
 		ControlFile->state != DB_SHUTDOWNED_IN_RECOVERY)
 	{