manual / manpages: Make option category names a proper subheader

Before they were an "ad-hoc" header with bold and a colon; now they are
a proper subheader.

For the man pages, this doesn't make much of a difference, but it will
help more on for the HTML manual, where things can be restyled. Again,
good separation of content vs presentation.

doc/manual/generate-manpage.nix

@@ -92,7 +92,7 @@ let
       showOptions = allOptions:
         let
           showCategory = cat: opts: ''
-            ${optionalString (cat != "") "**${cat}:**"}
+            ${optionalString (cat != "") "## ${cat}"}
 
             ${concatStringsSep "\n" (attrValues (mapAttrs showOption opts))}
             '';