More description fixes

* Remove package name
* Start with upper case letter
* Remove trailing period

Also reword some descriptions and move some long descriptions to
longDescription.

I'm not touching generated packages.

pkgs/tools/security/pwgen/default.nix

@@ -7,7 +7,7 @@ stdenv.mkDerivation {
     sha256 = "1afxbkdl9b81760pyb972k18dmidrciy3vzcnspp3jg0aa316yn8";
   };
   meta = {
-	description = "Small, GPL'ed password generator which creates passwords which can be easily memorized by a human.";
-        platforms = stdenv.lib.platforms.all;
+    description = "Password generator which creates passwords which can be easily memorized by a human";
+    platforms = stdenv.lib.platforms.all;
   };
 }