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/Xaw3d/default.nix

@@ -10,4 +10,8 @@ stdenv.mkDerivation {
   patches = [./config.patch ./laylex.patch];
   buildInputs = [x11 imake gccmakedep libXmu libXpm libXp bison flex];
   propagatedBuildInputs = [x11 libXmu];
+
+  meta = {
+    description = "3D widget set based on the Athena Widget set";
+  };
 }