Label the contents of pg_*_d.h files a little better.

Label the contents of pg_*_d.h files a little better.

Make genbki.pl emit some boilerplate comments identifying the
sections of the pg_*_d.h files that it generates.  This is in
hopes of making them slightly more readable, in case people
look at those files and not the pg_*.h/pg_*.dat originals.

Discussion: https://postgr.es/m/1134562.1742507765@sss.pgh.pa.us