Use COALESCE in update_counters docs to reflect the implementation

activerecord/lib/active_record/counter_cache.rb

@@ -56,15 +56,15 @@ def reset_counters(id, *counters)
     #   Post.update_counters 5, :comment_count => -1, :action_count => 1
     #   # Executes the following SQL:
     #   # UPDATE posts
-    #   #    SET comment_count = comment_count - 1,
-    #   #        action_count = action_count + 1
+    #   #    SET comment_count = COALESCE(comment_count, 0) - 1,
+    #   #        action_count = COALESCE(action_count, 0) + 1
     #   #  WHERE id = 5
     #
     #   # For the Posts with id of 10 and 15, increment the comment_count by 1
     #   Post.update_counters [10, 15], :comment_count => 1
     #   # Executes the following SQL:
     #   # UPDATE posts
-    #   #    SET comment_count = comment_count + 1,
+    #   #    SET comment_count = COALESCE(comment_count, 0) + 1,
     #   #  WHERE id IN (10, 15)
     def update_counters(id, counters)
       updates = counters.map do |counter_name, value|