[Testing guide] Use "Fixtures API" instead of "fixture api".
[Testing guide] Using "Fixtures API" consistently

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.
Fix grammar on ActiveRecord::AttributeMethods doc.

[ci skip]
Remove custom errors page section from the guides

This pattern is too problematic and introduces a lot of edge cases:

  * On 4.2, the issue https://github.com/rails/rails/issues/15124 is
    back again.
  * needs to define each action for each http status otherwise the
    router raises ActionController::RoutingError (No route matches).
  * If the router has `match "/*username",...` and some action is
    missing, Rails will pick up the "match" and try to do its job.
  * encourages people to copy & paste programming. Not DRY.

[ci skip]
Merge branch 'master' of github.com:rails/rails
Add docs for AS::TimeWithZone + and -

The example was taken from the commit message 676d6a6.

[ci skip]
Merge branch 'document-time-with-zone-plus-and-minus'
Better doc for AS::DateTime#seconds_since_midnight

Adds examples and keeps coherent with the documentation of the
similar method `seconds_until_end_of_day`. [ci skip]
[ci skip] Fix output of video_tag helper
Add foreign_type option for polymorphic has_one and has_many.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.
Add class level case operator support for error dispatching in Rescuable
Merge pull request #17970 from ulissesalmeida/foreign-type-has-many-has-one

Add foreign_type option for polymorphic has_one and has_many.
Fix ProtocolViolation/bind message supplies for polymorphic + pluck or group
Refactor `quoted_date`

Move microseconds formatting to `AbstractAdapter`.
Revert "Merge pull request #17943 from jeremywadsack/doc_cache_importability"

This reverts commit b0e7278ddbe483475d7b2e161e65ae18df91b9f8, reversing
changes made to 785d04e3109f69d0b9b9f4732179592f0ef04e52.

This is not true. It is a bug and we will fix in the latest 4.0.x
release.
Fix undesirable RangeError by Type::Integer. Add Type::UnsignedInteger.
Merge pull request #17998 from kamipo/refactor_quoted_date

Refactor `quoted_date`
Merge pull request #17793 from kamipo/fix_undesirable_range_error

Fix undesirable RangeError by Type::Integer. Add Type::UnsignedInteger.
Update guides with info about vendor/assets

fixes #17893

[ci skip]
Merge pull request #18000 from caike/master

Update guides with info about vendor/assets [ci skip]