Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.

pkgs/development/libraries/a52dec/default.nix

@@ -11,6 +11,7 @@ stdenv.mkDerivation rec {
   NIX_CFLAGS_COMPILE = "-fpic";
 
   meta = {
+    description = "ATSC A/52 stream decoder";
     homepage = http://liba52.sourceforge.net/;
   };
 }