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Remove outdated guide on using rack builder w/o rails middleware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
API Documentation Guidelines link correction [ci skip]
Raise RuntimeErrors with inspectable and MRI-independent messages.

Previous behaviour was MRI-dependent, now we're making sure the message
is correctly shown: something that can be relyied upon across every
Ruby implementation.
Properly fix Rubinius dependencies in Gemfile & Travis integration.

Before Rubinius 2.2.0, some parts of the stdlib were lazily loaded, even
if not included in the Gemfile. Now it's 100% required to include the
gem 'rubysl', otherwise the stdlib is not available entirely, breaking
everything.

Also, the rubysl-test-unit gem was depending on minitest ~> 4.7, thus causing
conflicts with Rails's dependency on minitest 5. That is not the case
anymore since rubysl-test-unit 2.0.2, so it is completely safe to
include it fully.

When Travis is going to update RVM to 1.24, it'll be safe to use rbx-2,
thus picking new versions automatically.
Merge pull request #13019 from razielgn/rbx-gemfile-and-travis-fixes

Rubinius Gemfile and travis fixes
Updated allow_failures for Rubinius 2.2.1.