Sync commentary in transam.h and bki.sgml.

Sync commentary in transam.h and bki.sgml.

Commit a6417078c missed updating some comments in transam.h about
reservation of high OIDs for development purposes.  Also tamp down
an over-optimistic comment there about how easy it'd be to change
FirstNormalObjectId.

Earlier, commit 09568ec3d failed to update bki.sgml for the split
between genbki.pl-assigned OIDs and those assigned during initdb.

Also fix genbki.pl so that it will complain if it overruns
that split.  It's possible that doing so would have no very bad
consequences, but that's no excuse for not detecting it.