autodoc: Add info to =for hackers lines in perlapi,intern

These lines have always given a file name for where an API element can
be found.  Now, it includes a line number; and, if different, where the
prototype definition is as well as the documentation.

Further, if multiple API elements are grouped together, each now has its
own information given there.

Note this information is not displayed to the user; it helps people
hacking on the core.

autodoc.pl

@@ -1628,20 +1628,69 @@ ($$$)
     print $fh "\n=over $description_indent\n";
     print $fh "\n=item C<${$_}->{name}>\n" for @items;
 
+    my @where_froms;
     my @deprecated;
     my @experimental;
     my @xrefs;
 
-    for my $item_ref (@items) {
-        my $item = $$item_ref;
+    for (my $i = 0; $i < @items; $i++) {
+        my $item = ${$items[$i]};
         my $name = $item->{name};
 
-        # If we're printing only a link to an element, this isn't the major
-        # entry, so no X<> here.
-        push @xrefs, $name unless $item->{flags} =~ /h/;
+        if ($item->{flags}) {
 
-        push @deprecated,   "C<$name>" if $item->{flags} =~ /D/;
-        push @experimental, "C<$name>" if $item->{flags} =~ /x/;
+            # If we're printing only a link to an element, this isn't the major
+            # entry, so no X<> here.
+            push @xrefs, $name unless $item->{flags} =~ /h/;
+
+            push @deprecated,   "C<$name>" if $item->{flags} =~ /D/;
+            push @experimental, "C<$name>" if $item->{flags} =~ /x/;
+        }
+
+        # While we're going though the items, construct a nice list of where
+        # things are declared and documented.  Spend a bit of time to make it
+        # easier on the humans who later read it.
+        my $entry = "";
+        if ($item->{proto_defined}) {
+            $entry .= "declared "
+                   .  where_from_string($item->{proto_defined}{file},
+                                        $item->{proto_defined}{line_num});
+        }
+
+        if ($item->{docs_found}) {
+            if ($i == 0) {
+                # Special consolidated text if a single item in the list, and
+                # it's declared and documented in the same place.
+                if (   $entry
+                    && @items == 1
+                    && $item->{proto_defined}
+                    && $item->{docs_found}
+                    && $item->{proto_defined}{file} eq $item->{docs_found}{file}
+                    && $item->{proto_defined}{line_num} eq
+                                                $item->{docs_found}{line_num})
+                {
+                    $entry =~ s/declared/declared and documented/;
+                }
+                else {
+                    $entry .= "; " if $entry;
+                    $entry .= "all in group " if @items > 1;
+                    $entry .= "documented "
+                        .  where_from_string($item->{docs_found}{file},
+                                             $item->{docs_found}{line_num});
+                }
+            }
+        }
+
+        # If there is a single item in the group, no need to give the name,
+        # bu`t capitalize the first word.
+        if (@items == 1) {
+            $entry = ucfirst($entry);
+        }
+        else {
+            $entry = "$name $entry";
+        }
+
+        push @where_froms, $entry;
     }
 
     print $fh format_pod_indexes(\@xrefs);
@@ -2068,7 +2117,9 @@ ($$$)
     }
 
     print $fh "\n=back\n";
-    print $fh "\n=for hackers\nFound in file $file\n";
+    print $fh "\n=for hackers\n";
+    print $fh join "\n", @where_froms;
+    print $fh "\n";
 }
 
 sub construct_missings_section {