[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
Add class level case operator support for error dispatching in Rescuable
Make AC::Params.permit_all_parameters thread safe

As discussed in #16299[1], this attribute is not thread safe and could
potentially create a security issue.

[1]: https://github.com/rails/rails/pull/16299#discussion_r15424533
Add AC::Parameters#to_unsafe_h

As suggested in #16299([1]), this method should be a new public API for
retrieving unfiltered parameters from `ActionController::Parameters`
object, given that `Parameters#to_hash` will no longer work in Rails
5.0+ as we stop inheriting `Parameters` from `Hash`.

[1]: https://github.com/rails/rails/pull/16299#issuecomment-50220919
Fix default queue name prefix delimiter

Now displays as underscore, and not an empty string, when converted from markdown to HTML.
Escaped the underscore per http://daringfireball.net/projects/markdown/syntax#em
Fixes rails/rails#18009
Merge pull request #18010 from audionerd/patch-1

Fix default queue name prefix delimiter
Merge pull request #18003 from sikachu/permit_all_parameters-thread-safety

Make AC::Params.permit_all_parameters thread safe
Align the GemfileEntry API with stable branches

See 63462ec
Rails master requires arel master
Improving tips about installation in the guides [ci skip]
New applications should use sass-rails 5.0
Pass through the `prepend` option to `AS::Callback`

I'm not sure what's the use case for this, but apparently it broke some apps.
Since it was not the intended result from #16210 I fixed it to not raise an
exception anymore. However, I didn't add documentation for it because I don't
know if this should be officially supported without knowing how it's meant to
be used.

In general, validations should be side-effect-free (other than adding to the
error message to `@errors`). Order-dependent validations seems like a bad idea.

Fixes #18002
Mention that Action Mailer methods are not deferred [ci skip]

Closes #17965, #17968

[Godfrey Chan, Miklós Fazekas]