signal-desktop: Refactor the package description

Removed some redundancy (src check via meta.platforms) and made some
changes according to our style-guide.
I've changed meta.description and added meta.longDescription.

pkgs/applications/networking/instant-messengers/signal-desktop/default.nix

@@ -1,8 +1,10 @@
-{ stdenv, lib, fetchurl, dpkg, wrapGAppsHook, gnome3, gtk3, atk, cairo, pango, gdk_pixbuf, glib, freetype,
-fontconfig, dbus, libX11, xorg, libXi, libXcursor, libXdamage, libXrandr,
-libXcomposite, libXext, libXfixes, libXrender, libXtst, libXScrnSaver, nss,
-nspr, alsaLib, cups, expat, udev
+{ stdenv, lib, fetchurl, dpkg, wrapGAppsHook
+, gnome3, gtk3, atk, cairo, pango, gdk_pixbuf, glib, freetype, fontconfig
+, dbus, libX11, xorg, libXi, libXcursor, libXdamage, libXrandr, libXcomposite
+, libXext, libXfixes, libXrender, libXtst, libXScrnSaver, nss, nspr, alsaLib
+, cups, expat, udev
 }:
+
 let
   rpath = lib.makeLibraryPath [
     alsaLib
@@ -36,24 +38,21 @@ let
     xorg.libxcb
   ];
 
-in
-  stdenv.mkDerivation rec {
+in stdenv.mkDerivation rec {
   name = "signal-desktop-${version}";
-
   version = "1.12.0";
 
-    src =
-      if stdenv.system == "x86_64-linux" then
-        fetchurl {
+  src = fetchurl {
     url = "https://updates.signal.org/desktop/apt/pool/main/s/signal-desktop/signal-desktop_${version}_amd64.deb";
     sha256 = "19c9pilx2qkpyqw3p167bfcizrpd4np5jxdcwqmpbcfibmrpmcsk";
-        }
-      else
-        throw "Signal for Desktop is not currently supported on ${stdenv.system}";
+  };
 
   phases = [ "unpackPhase" "installPhase" ];
+
   nativeBuildInputs = [ dpkg wrapGAppsHook ];
+
   unpackPhase = "dpkg-deb -x $src .";
+
   installPhase = ''
     mkdir -p $out
     cp -R opt $out
@@ -81,12 +80,14 @@ in
   '';
 
   meta = {
-      description = "Signal Private Messenger for the Desktop.";
+    description = "Private, simple, and secure messenger";
+    longDescription = ''
+      Signal Desktop is an Electron application that links with your
+      "Signal Android" or "Signal iOS" app.
+    '';
     homepage    = https://signal.org/;
     license     = lib.licenses.gpl3;
     maintainers = with lib.maintainers; [ ixmatus primeos ];
-      platforms   = [
-        "x86_64-linux"
-      ];
+    platforms   = [ "x86_64-linux" ];
   };
 }