autodoc: Allow specifying a section may be empty

There are lots of empty sections in perlintern, but until this commit it
was assumed that all sections would have something in perlapi.  This
allows the user to specify that particular sections may not have perlapi
entries.

Author: khwilliamson

autodoc.pl

@@ -2310,10 +2310,14 @@ ($destpod)
     for my $section_name (sort dictionary_order keys %valid_sections) {
         my $section_info = $dochash->{$section_name};
 
-        # We allow empty sections in perlintern.
-        if (! $section_info && $podname eq $api) {
-            warn "Empty section '$section_name' for $podname; skipped";
-            next;
+        if (! $section_info) {
+            # We always allow empty sections in perlintern.
+            if (   $podname eq $api
+                && ! $valid_sections{$section_name}{may_be_empty_in_perlapi})
+            {
+                warn "Empty section '$section_name' for $podname; skipped";
+                next;
+            }
         }
 
         print $fh "\n=head1 $section_name\n";