[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
fixing mailer previews for apps with globbing route
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] add `enable_extension` to PostgreSQL hstore example
Test coverage for ActiveJob generator
Merge pull request #17950 from y-yagi/postgres_guide

[ci skip] add `enable_extension` to PostgreSQL hstore example
Fix grammar of sentence in Reflection documentation
Merge pull request #17959 from eileencodes/fix-grammar-on-reflection

Fix grammar of sentence in Reflection documentation [ci skip]
[ci skip] Fix output of video_tag helper
minor sentences fixes
Make error message clearer that :on requires a symbol, not a string

The validation added in 5a3dc8092d19c816b0b1203945639cb91d065847 will
reject values for the `:on` option for after_commit and after_rollback
callbacks that are string values like `"create"`.

However, the error message says ":on conditions for after_commit and
after_rollback callbacks have to be one of create,destroy,update". That
looks like a string value *would* be valid.

This commit changes the error message to say ":on conditions for
after_commit and after_rollback callbacks have to be one of [:create,
:destroy, :update]", making it clearer that symbols are required.
Merge pull request #17963 from neerajdotname/wordsmitthing

minor sentences fixes
Add note to Store about uniqueness validation (#17954) [skip ci]
Merge pull request #17953 from kirs/test-activejob-generator

Test coverage for ActiveJob generator