[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
[ci skip] Fix broken url in plugins guide
Merge pull request #16472 from y-yagi/patch-1

[ci skip] Fix broken url in plugins guide
users_dont_suck_but_only_we_suck_and_only_our_tests_are_order_dependent!

Calling ActiveSupport::TestCase.i_suck_and_my_tests_are_order_dependent! in AS::TestCase makes
everyone's tests order dependent, which should never be done by the framework.
Remove old setup from AS test case

This was added back in Rails 3 on
c4a6109286909c394e8c5bfc471a1eb9de245d2b, and is not being used anymore.
CHANGELOG entry for TestCase.i_suck_and_my_tests_are_order_dependent! fix
use 'based on' instead of 'based off' [ci skip]
Nobody sucks so nobody should call this awful method name
Merge pull request #16476 from akshay-vishnoi/doc-changes

[ci skip] use "based on" or "based off of"
:bomb:
Fix `touch: true` in polymorphic relations

Makes the `has_many :as` end of the relationship having an inverse.