Small Action View Overview edits [ci skip]

Some small grammar and readability edits
Define the Duration#instance_of? method

Since Duration is extending from ProxyObject which extends itself from
BasicObject, the Duration object doesn't respond to the #instance_of?
method. Thus, the #method_missing hook get triggered, delegating the
method to its `value` attribute.

However, Rubinius' #eql? definition relies on #instance_of?, thus this
will equal to true with a Fixnum (since its `value` attribute is a
Fixnum) while it should not.

The previous behavior was wrong anyway, no matter the implementation.
Grammar pass

[ci skip]

Add improvements from @eileencodes [skip ci]
Added enqueue_at support for QueAdapter in ActiveJob

* Added inline job runner for Que in test/support
* Updated QueAdapter to support enqueue_at
Change 'does' to 'do'
Merge pull request #16603 from jwworth/task/fix_grammar

[ci skip] 4.2 release notes grammar fix
Upgrade guide pass. [CI SKIP]
Avoid duplicating routes for HEAD requests.

Follow up to rails#15321

Instead of duplicating the routes, we will first match the HEAD request to
HEAD routes. If no match is found, we will then map the HEAD request to
GET routes.
Improve router test.

We should assert that routes will not be recognized if the verbs do
not match.
Merge pull request #15443 from tgxworld/preload_head_routes

Map HEAD requests to GET routes instead of duplicating GET routes.
Fix some issues in 4.2 release notes.

Remove duplicate line in Railties.

[ci skip]
Merge pull request #16608 from GRoguelon/fix_issues_in_4_2_release_notes

Fix some issues in 4.2 release notes. [ci skip]
Merge pull request #16607 from tgxworld/upgrading_guide_pass

Upgrade guide pass. [CI SKIP]
[ci skip] Clarify Action Mailer/Active Job usage note
Merge pull request #16612 from loganhasson/master

[ci skip] Clarify Action Mailer/Active Job usage note
Make text consistent in help text of rails server and console commands.
Guide fix. [CI SKIP]

Related: https://github.com/rails/rails/pull/16607
adding the exception RecordNotFound to UUID find
[ci skip] Added documentation for has_one scope parameter
Merge pull request #16615 from tgxworld/guide_fix

[ci skip] Grammar fixes for upgrade guide from 4.0 to 4.1