[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.
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