Clarify deprecation warning.

lib/geocoder/results/nominatim.rb

@@ -80,7 +80,7 @@ module Geocoder::Result
     end
 
     define_method 'class' do
-      warn "Method 'class' is deprecated. Use 'place_class' instead"
+      warn "DEPRECATION WARNING: The 'class' method of Geocoder::Result::Nominatim objects is deprecated and will be removed in Geocoder version 1.2.0. Please use 'place_class' instead."
       @data['class']
     end