Explain dropdb can't use syscache because of TOAST

Add a comment explaining dropdb() can't rely on syscache. The issue with
flattened rows was fixed by commit 0f92b230f8, but better to have
a clear explanation why the systable scan is necessary. The other places
doing in-place updates on pg_database have the same comment.

Suggestion and patch by Yugo Nagata. Backpatch to 12, same as the fix.

Author: Yugo Nagata
Backpatch-through: 12
Discussion: https://postgr.es/m/CAJTYsWWNkCt+-UnMhg=BiCD3Mh8c2JdHLofPxsW3m2dkDFw8RA@mail.gmail.com

src/backend/commands/dbcommands.c

@@ -962,7 +962,9 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 	dropDatabaseDependencies(db_id);
 
 	/*
-	 * Update the database's pg_database tuple
+	 * Get the pg_database tuple to scribble on.  Note that this does not
+	 * directly rely on the syscache to avoid issues with flattened toast
+	 * values for the in-place update.
 	 */
 	ScanKeyInit(&scankey,
 				Anum_pg_database_datname,