$docdev fixups: mostly to avoid empty output

... after auto-removing some kinds of files by default.
In some cases I let them be removed and in others I let them be put into
$docdev. That was more due to general indecisiveness on this question
than any reasons in the particular cases.

pkgs/development/libraries/libtheora/default.nix

@@ -8,7 +8,8 @@ stdenv.mkDerivation rec {
     sha256 = "0swiaj8987n995rc7hw0asvpwhhzpjiws8kr3s6r44bqqib2k5a0";
   };
 
-  outputs = [ "dev" "out" "doc" ]; # dev-doc only
+  outputs = [ "dev" "out" "docdev" ];
+  outputDoc = "docdev";
 
   nativeBuildInputs = [ pkgconfig ];
   propagatedBuildInputs = [ libogg libvorbis ];