[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]
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
Don't convert empty arrays to nils when deep munging params
autoloading guide: adds nesting rule for Kernel#load [ci skip]
Already deprecated [ci skip]
fix a typo [ci skip]
fix a typo [ci skip]
Merge branch 'master' of github.com:rails/docrails
Merge pull request #18038 from seuros/typofix

fix a typo [ci skip]
Merge pull request #18041 from andreynering/ar-guides-querying-2

Dynamic finders already deprecated [ci skip]
autoloading guide: applies some Strunk & White and hyperlinking [ci skip]
Merge pull request #16924 from Sinjo/params-deep-munge-empty-array

Don't convert empty arrays to nils when deep munging params
Revert "Re-enable capture'ing non-String values"

This reverts commit c2fe0938d7201d4ce0bb2f25e72bf5f70df128af.
Regression test for #18024