[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
Improving tips about installation in the guides [ci skip]
Fix Duplicate web-console entries in Gemfile
Add test for `:skip_after_callbacks_if_terminated`

`define_callbacks` from `ActiveSupport::Callbacks` accepts the
`:skip_after_callbacks_if_terminated` option since #4866 but the option
is not tested anywhere.

This commit adds tests and fixes documentation for the option, making it clear
that halting a callback chain only stops following `before_` and `around_`
callbacks by default.
Add AM test for after_validation returning false

This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358

It's simply a clarification of the current behavior by which if an
`after_validation` ActiveModel callback returns +false+, then further
`after_` callbacks **are not halted**.
Add AM test: after/around callback returning false

This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358

It's simply a clarification of the current behavior by which if an
`after_` or `around_` ActiveModel callback returns +false+, then the callback
chain **is not halted**.

The callback chain in ActiveModel is only halted when a `before_`
callback returns `false`.
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]