[ci skip] Mention the change in nil handling for serialized attributes
in "Upgrading from Rails 3.2 to Rails 4.0"
[ci skip] Clarify the "Serialized Attributes" section

Mention that the change in nil handling for serialized attributes
affects custom coders.
Merge pull request #16300 from vast/mention-changes-to-nil-serialization-in-upgrading-from-3.2

Mention changes to `nil` handling in serialized attributes