CI: .travis.yml: add description to the channels

M  .travis.yml

.travis.yml

@@ -29,9 +29,9 @@ env:
     # { master
     # , commitHash
     # , haskell-updates  # Haskell development branch in Nixpkgs, can be inconsistent. Weekly merged into the upstream
-    # , nixpkgs-unstable
-    # , nixos-unstable
-    # , nixos-20.03
+    # , nixpkgs-unstable  # Default branch on Nix installation, default for non NixOS
+    # , nixos-unstable  # nixpkgs-unstable that passes a bunch of base tests
+    # , nixos-20.03  # Last stable release, gets almost no updates to recipes, gets only required backports
     # }
     # - rev=nixos-unstable
     # # NOTE: Switching into Nixpkgs revision