Update README.rdoc [ci skip]

Highlighted code
Update commands.rb content in initialization guide

[ci skip]
Add section for command_tasks in initialization guide

[ci skip]
Change syntax format for example returned values

According to our guideline, we leave 1 space between `#` and `=>`, so we
want `# =>` instead of `#=>`.

Thanks to @fxn for the suggestion.

[ci skip]
Fix RailsServer#start content in initialization guide

[ci skip]
[ci skip] Replace #=> with # =>
Correct guide relating MiddlewareStackProxy with Enumerable

[ci skip] Enumerable.instance_methods &
Rails::Configuration::MiddlewareStackProxy.instance_methods # =>
[:as_json]

MiddlewareStackProxy does not share any notable methods with Enumerable.
Remove outdated guide on using rack builder w/o rails middleware

[ci skip]
Fix broken Wikipedia link.
Update action_controller_overview.md [ci skip]

Code style adherence
The option is called encode_big_decimal_as_string [ci-skip]
syntax error joining tables

syntax error joining tables
syntax error joining/including models

syntax error joining/including models
Revert last two commits mistakenly changing join/include syntax

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Revert "syntax error joining/including models"

This reverts commit ac35f72f30c47298a20a90c8c537db888b463bbb.

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Revert "syntax error joining tables"

This reverts commit c365986b48c3e8bc8c7f3fa6a8521616ed5dc138.

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Comments:

https://github.com/rails/docrails/commit/c365986b48c3e8bc8c7f3fa6a8521616ed5dc138#commitcomment-4630684
Improve reading / style of hashes in AR guide
Fix the assertion for not inclusion of turbolinks on application.js.

We were asserting if `//= turbolinks` was not present while the correct is
asserting that `//= require turbolinks` is not present.
Merge pull request #13003 from laurocaetano/fix_assertion_for_turbolinks

Fix the assertion for not inclusion of turbolinks on application.js.
Use a more generic assertion to make sure this tests will be useful.

Since we are using assert_no_match, if we change the code before
changing the tests, the tests will still pass and the assertion will
become useless
Fix validation on uniqueness of empty association
When Array#as_json and Hash#as_json are called without options, they
should also call #as_json on the children without options (instead of
nil)