[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 class level case operator support for error dispatching in Rescuable
Fix ProtocolViolation/bind message supplies for polymorphic + pluck or group
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]
Merge pull request #17994 from mfazekas/pluck-bind-values-master

Fix ProtocolViolation/bind message for polymorphic + pluck or group+calc
Improve the test case introduced by bd0d47e
Add test case for joined pluck

https://github.com/rails/rails/commit/39542fba54328ca048fb75a5d5b37f8e1d4c1f37#commitcomment-8938379
Merge pull request #17815 from yuki24/remove-custom-errors-page-section

Remove custom errors page section from the guides [ci skip]
Make AC::Params.permit_all_parameters thread safe

As discussed in #16299[1], this attribute is not thread safe and could
potentially create a security issue.

[1]: https://github.com/rails/rails/pull/16299#discussion_r15424533
Add AC::Parameters#to_unsafe_h

As suggested in #16299([1]), this method should be a new public API for
retrieving unfiltered parameters from `ActionController::Parameters`
object, given that `Parameters#to_hash` will no longer work in Rails
5.0+ as we stop inheriting `Parameters` from `Hash`.

[1]: https://github.com/rails/rails/pull/16299#issuecomment-50220919
English fix [ci skip]