Also add doc examples for `Object.nil`.

[ci skip]

Replace guides.ror.org/v4.2.0 with guides.ror.org

No need to point links to http://guides.rubyonrails.org/v4.2.0/... now
that the Ruby on Rails guides point to 4.2.0.

Fix link in autoloading guide

[ci skip]

Add link for Domain-Specific Language [ci skip]

Make bin/setup portable

Add nodoc to some constants [skip ci]

Allow static asset serving from env variable (enhanced!)
Conflicts:
	railties/CHANGELOG.md

`db:schema:load` and `db:structure:load` do not purge the database

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446

This reverts commit da5cc10e.

Fixes #18091

See also https://github.com/rails/rails/pull/18003#commitcomment-9030909

/cc @senny Is this change documented somewhere?

Add docs for `extract_domain`, `extract_subdomains`, `extract_subdomain`.

Add doc examples for `url`, `protocol`, `raw_host_with_port`, `host`,
`host_with_port`, `port`, `standard_port`, `standard_port?`, `optional_port`,
`port_string`.

[ci skip]

And also bundler cache

Add docs for AS::TestCase::test_order

Document `test_order` and `test_order=` from `ActiveSupport::TestCase`.

[ci skip]

Spelling fix [ci skip]

/cc @matthewd

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems

Apparently PG does not validate against RFC 4122. The intent of the original
patch is just to protect against PG errors (which potentially breaks txns, etc)
because of bad user input, so we shouldn't try any harder than PG itself.

Closes #17931

[ci skip] Fix HTML format on comment

Add docs for `minutes`, `hours`, `days`, `weeks` and `fortnights`.
Fix docs for `in_milliseconds`.

[ci skip]

@rafaelfranca suggested in f7c7bcd9 that code examples should display
the result after  `# =>` and not after `#=>`.

This commit replaces *all* the occurrences of `#=>` in the code documentation
(mostly added by me 😭) with the suggested `# =>`.

Add docs for `kilobytes`, `megabytes`, `gigabytes`, `terabytes`,
`petabytes` and `exabytes`. Fix docs for `bytes`.

[ci skip]

Also improves docs for `Time#find_zone!`

[ci skip]

Fix name on changelog [ci skip]