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

src/backend/catalog/genbki.pl

@@ -480,6 +480,8 @@ foreach my $catname (@catnames)
 
 EOM
 
+	printf $def "/* Macros related to the structure of $catname */\n\n";
+
 	# Emit OID macros for catalog's OID and rowtype OID, if wanted
 	printf $def "#define %s %s\n",
 	  $catalog->{relation_oid_macro}, $catalog->{relation_oid}
@@ -561,6 +563,7 @@ EOM
 	print $def "\n#define Natts_$catname $attnum\n\n";
 
 	# Emit client code copied from source header
+	printf $def "/* Definitions copied from ${catname}.h */\n\n";
 	foreach my $line (@{ $catalog->{client_code} })
 	{
 		print $def $line;
@@ -573,6 +576,9 @@ EOM
 		print $bki "open $catname\n";
 	}
 
+	printf $def
+	  "\n/* OID symbols for objects defined in ${catname}.dat */\n\n";
+
 	# For pg_attribute.h, we generate data entries ourselves.
 	if ($catname eq 'pg_attribute')
 	{