[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.
[ci skip] Fix sample code in engines guide
Merge pull request #16451 from y-yagi/patch-2

[ci skip] Fix sample code in engines guide
Cleanup Guides Guidelines [ci skip]

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames
Merge pull request #16453 from reshleman/re-cleanup-guides-guidelines

Cleanup Guides Guidelines [ci skip]
Merge pull request #16300 from vast/mention-changes-to-nil-serialization-in-upgrading-from-3.2

Mention changes to `nil` handling in serialized attributes
update error message to reflect that the record could have been destroyed
Merge pull request #16456 from lsylvester/improve-active-record-error-message-on-update_columns-for-destroyed-records

Change error message in update_columns for destroyed objects
Fix counter_cache for polymorphic associations

Also removes a false positive test that depends on the fixed bug:

At this time, counter_cache does not work with polymorphic relationships
(which is a bug). The test was added to make sure that no
StaleObjectError is raised when the car is destroyed. No such error is
currently raised because the lock version is not incremented by
appending a wheel to the car.

Furthermore, `assert_difference` succeeds because `car.wheels.count`
does not check the counter cache, but the collection size. The test will
fail if it is replaced with `car.wheels_count || 0`.