[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]
allow 'all' for :domain option in addition to :all
Don't remove mailer layouts files
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]
Fix Sidekiq ActiveJob integration setup

* .connect on a Redis connection wasn't valid
* Reset logger after we're done testing for redis connection to avoid
  "closed stream" error when starting server for real from a fork
Hide potentially sensitive ActiveJob params from logs

* Show GlobalID instead of full object .inspect output
Merge branch 'master' of github.com:rails/rails
Only filter GIDable objects; remove cosmetic changes
Require missing association in test
Fix "nonexistent" typo in tests
remove useless methods
Merge pull request #17889 from mxie/mx-fix-nonexistent-typo

Fix "nonexistent" typo in tests
Removed use of mocha in the info_controller tests
[ci skip] remove references to `Numeric#from_now`
Merge pull request #17892 from y-yagi/active_support_core_extensions_guide

[ci skip] remove references to `Numeric#from_now`
Merge pull request #17890 from sergey-alekseev/remove-useless-methods

remove useless methods
skip test to get `sqlite3_mem` suite passing.
fixing mailer previews for apps with globbing route