Add a comment in init.h to remind devs to update makedoc.c when adding
a new type.

Change makedoc to fail on a missing type.  Otherwise it's too easy to
miss the resulting problem in the documentation.

Credit goes to Eike Rathke for noting that it was too easy to forget
to do this, and something should be fixed.  I found his solution a bit
too complicated though, and opted for just adding comment and program
failure.