xorg: Split some man outputs from doc

Because soon manpages won't go to $doc anymore.

pkgs/servers/x11/xorg/overrides.nix

···
   libxcb = attrs : attrs // {
     nativeBuildInputs = [ args.python ];
     configureFlags = "--enable-xkb --enable-xinput";
-    outputs = [ "out" "dev" "doc" ];
+    outputs = [ "out" "dev" "man" "doc" ];
   };
 
   xcbproto = attrs : attrs // {
···
   };
 
   libXext = attrs: attrs // {
-    outputs = [ "out" "dev" "doc" ];
+    outputs = [ "out" "dev" "man" "doc" ];
     propagatedBuildInputs = [ xorg.xproto xorg.libXau ];
     preConfigure = setMalloc0ReturnsNullCrossCompiling;
   };
···
   };
 
   libXi = attrs: attrs // {
-    outputs = [ "out" "dev" "doc" ];
+    outputs = [ "out" "dev" "man" "doc" ];
     propagatedBuildInputs = [ xorg.libXfixes ];
   };