Fix comments on wal_level=minimal, CREATE TABLESPACE and CREATE DATABASE.

Fix comments on wal_level=minimal, CREATE TABLESPACE and CREATE DATABASE.

Commit 97ddda8a82ac470ae581d0eb485b6577707678bc removed the rmtree()
behavior from XLOG_TBLSPC_CREATE, obsoleting that part of the comment.
The comment's point about XLOG_DBASE_CREATE was wrong when commit
fa0f466d5329e10b16f3b38c8eaf5306f7e234e8 introduced the point.  (It
would have been accurate if that commit had predated commit
fbcbc5d06f53aea412130deb52e216aa3883fb8d introducing the second
checkpoint of CREATE DATABASE.)  Nothing can skip log_smgrcreate() on
the basis of wal_level=minimal, so don't comment on that.

Commit c6b92041d38512a4176ed76ad06f713d2e6c01a8 expanded WAL skipping
from five specific operations to relfilenodes generally, hence the
CreateDatabaseUsingFileCopy() comment change.

Discussion: https://postgr.es/m/20231008022204[email protected]