nixos/xserver: Document xrandrHeads.apply

It was asked by @CMCDragonkai to elaborate on that, so let's just do
this by actually providing a code comment.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/services/x11/xserver.nix

@@ -374,6 +374,8 @@ in
         type = with types; listOf (coercedTo str (output: {
           inherit output;
         }) (submodule { options = xrandrOptions; }));
+        # Set primary to true for the first head if no other has been set
+        # primary already.
         apply = heads: let
           hasPrimary = any (x: x.primary) heads;
           firstPrimary = head heads // { primary = true; };