[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
Fix Duplicate web-console entries in Gemfile
Add `null` option to timestamps as migration generator does [ci skip]
Merge pull request #18030 from zzak/doc-timestamps-null-option-from-generator

Add `null` option to timestamps as migration generator does [ci skip]
autoloading guide, fix typo `Amin` -> `Admin`. [ci skip]

/cc @fxn
autoloading guide: relax type of constant names [ci skip]
autoloading guide: new section about why Kernel#autoload is not involved [ci skip]
autoloading guide: Kernel#autoload -> Module#autoload [ci skip]
autoloading guide: minor edits.

/cc @fxn
autoloading guide: documents nesting
autoloading guide: damn typos [ci skip]
autoloading guide: documents constant resolution algorithms
autoloading guide: more edits [ci skip]