Minor tweaks to user map/scenario default descriptions

* Added period to "No description available." Technically this is just the default no-description text, but it
  appears most often for custom scenarios.
* "User made map" -> "Custom map."

src/game_initialization/create_engine.cpp

@@ -146,7 +146,7 @@ std::string user_map::description() const
 	}
 
 	// map error message
-	return _("User made map");
+	return _("Custom map.");
 }
 
 random_map::random_map(const config& data)

src/gui/dialogs/multiplayer/mp_create_game.cpp

@@ -501,7 +501,7 @@ void mp_create_game::show_description(window& window, const std::string& new_des
 {
 	styled_widget& description = find_widget<styled_widget>(&window, "description", false);
 
-	description.set_label(!new_description.empty() ? new_description : _("No description available"));
+	description.set_label(!new_description.empty() ? new_description : _("No description available."));
 	description.set_use_markup(true);
 }