Merge pull request #16086 from nickpellant/master

Add callbacks documentation for upgrading to 4.1.

guides/source/upgrading_ruby_on_rails.md

@@ -432,6 +432,20 @@ symbol access is no longer supported. This is also the case for
 `store_accessors` based on top of `json` or `hstore` columns. Make sure to use
 string keys consistently.
 
+### Explicit block use for `ActiveSupport::Callbacks`
+
+Rails 4.1 now expects an explicit block to be passed when calling 
+`ActiveSupport::Callbacks.set_callback`. This change stems from 
+`ActiveSupport::Callbacks` being largely rewritten for the 4.1 release.
+
+```ruby
+# Previously in Rails 4.0
+set_callback :save, :around, ->(r, &block) { stuff; result = block.call; stuff }
+
+# Now in Rails 4.1
+set_callback :save, :around, ->(r, block) { stuff; result = block.call; stuff }
+```
+
 Upgrading from Rails 3.2 to Rails 4.0
 -------------------------------------