youtube-dl: clean up the expression and document the pandoc situation

This patch resolves https://github.com/NixOS/nixpkgs/pull/8184.

pkgs/tools/misc/youtube-dl/default.nix

@@ -1,9 +1,11 @@
-{ manuals ? true
-, stdenv, fetchurl, makeWrapper, python, zip, 
-pandoc ? null 
-, ffmpeg }:
+{ stdenv, fetchurl, makeWrapper, python, zip, ffmpeg, pandoc ? null }:
 
-assert manuals -> pandoc != null;
+# Pandoc is required to build the package's man page. Release tarballs
+# contain a formatted man page already, though, so it's fine to pass
+# "pandoc = null" to this derivation; the man page will still be
+# installed. We keep the pandoc argument and build input in place in
+# case someone wants to use this derivation to build a Git version of
+# the tool that doesn't have the formatted man page included.
 
 stdenv.mkDerivation rec {
   name = "youtube-dl-${version}";
@@ -14,14 +16,9 @@ stdenv.mkDerivation rec {
     sha256 = "0lgxir2i5ipplg57wk8gnbbsdrk7szqnyb1bxr97f3h0rbm4dfij";
   };
 
-  nativeBuildInputs = [ ]
-    ++ stdenv.lib.optional manuals pandoc;
+  buildInputs = [ python makeWrapper zip pandoc ];
 
-  buildInputs = [ python makeWrapper zip ];
-
-  patchPhase = ''
-    rm youtube-dl
-  '';
+  patchPhase = "rm youtube-dl";
 
   configurePhase = ''
     makeFlagsArray=( PREFIX=$out SYSCONFDIR=$out/etc PYTHON=${python}/bin/python )

pkgs/top-level/all-packages.nix

@@ -3385,8 +3385,10 @@ let
   yi = callPackage ../applications/editors/yi/wrapper.nix { };
 
   youtube-dl = callPackage ../tools/misc/youtube-dl {
-  	manuals = false;
-   };
+    # Release versions don't need pandoc because the formatted man page
+    # is included in the tarball.
+    pandoc = null;
+  };
 
   zbar = callPackage ../tools/graphics/zbar {
     pygtk = lib.overrideDerivation pygtk (x: {