integrate the strong params README into the AC guide.

The current ActionController guide does not mention strong parameters
at all. I integrated the README into the guide to explain the API.

I also included a section to illustrate that the API does not solve
all possible whitelisting scenarios.

The origin was #9454.
Merge pull request #9465 from senny/document_strong_params_in_ac_guide

integrate the strong params README into the AC guide.
fix the sql that is generated from scoping
Use custom visitor class for optimized url helpers

Rather than trying to use gsub to remove the optional route segments,
which will fail with nested optional segments, use a custom visitor
class that returns a empty string for group nodes.

Closes #9524
Bring config.allow_concurrency back

Since the Rack::Lock still exists in development,
let's provide a way to disable it explicitly.
Created an unscope method for removing relations from a chain of
relations. Specific where values can be unscoped, and the unscope method
still works when relations are merged or combined.
Merge pull request #9532 from neerajdotname/fix-sql-in-example

In example fix sql that is generated from scoping
Merge pull request #9200 from wangjohn/unscoping_activerecord_merging

Introduce relation #unscope
add prompt to 422 page to check logs
Merge pull request #9537 from NARKOZ/patch-6

add prompt to 422 page to check logs
Use 1.9 hash style in changelog and doc examples [ci skip]
Remove .all usage from unscope doc examples [ci skip]
Fix formats on xhr requests when HTTP_ACCEPT is empty string

Fix ActionDispatch::Request#formats on xhr requests when HTTP_ACCEPT
header is empty string. About issue #7774, same fix as in commit bebb02f
but for xhr requests.
Fixed typo in active record's changelog
Merge pull request #9540 from dimko/patch-1

Fixed typo in active record's changelog
Merge pull request #9541 from maximerety/fix_7774

Fix formats on xhr requests when HTTP_ACCEPT is empty string
only calculate next_id when we need a new method defined
rewrite tests to have no side-effects on `include_root_in_json`.
`ActiveRecord::Base.include_root_in_json` is `false` by default.

Closes #9459.

The PR #6597 unified the configuration for `include_root_in_json`
in AM and AR to `false`.
Later on with the refactoring commit: e030f26 the value in AR was
set to `true` but I think this was not on purpose.

With this commit both AM and AR will have the same configuration
for `include_root_in_json`, which is `false`.
Merge pull request #9467 from senny/9459_include_json_root_out_of_sync

`ActiveRecord::Base.include_root_in_json` is `false` by default.