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
0f92b230f88b, 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