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Remove outdated guide on using rack builder w/o rails middleware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
Use `SchemaStatements#initialize_schema_migrations_table` instead of `ActiveRecord::SchemaMigration.create_table`.
swap arguments for some `assert_equal` statements => `expected, actual`
Add positional information to eval call so that this information will
be used in printing correct location where the exception occurred.
Closes #12885

- Without this the location of exception is always the line on which
  'eval' is called
- But if the exception occurs in a gem outside of Rails, then that
  location is not printed in stacktrace
Fix ActiveRecord::Relation#unscope

I'm pretty confused about the addition of this method. The documentation
says that it was intended to allow the removal of values from the
default scope (in contrast to #except). However it behaves exactly the
same as except: https://gist.github.com/jonleighton/7537008 (other than
having a slightly enhanced syntax).

The removal of the default scope is allowed by
94924dc32baf78f13e289172534c2e71c9c8cade, which was not a change we
could make until 4.1 due to the need to deprecate things. However after
that change #unscope still gives us nothing that #except doesn't already
give us.

However there *is* a desire to be able to unscope stuff in a way that
persists across merges, which would allow associations to be defined
which unscope stuff from the default scope of the associated model. E.g.

  has_many :comments, -> { unscope where: :trashed }

So that's what this change implements. I've also corrected the
documentation. I removed the guide references to #except as I think
unscope really supercedes #except now.

While we're here, there's also a potential desire to be able to write
this:

  has_many :comments, -> { unscoped }

However, it doesn't make sense and would not be straightforward to
implement. While with #unscope we're specifying exactly what we want to
be removed from the relation, with "unscoped" we're just saying that we
want it to not have some things which were added earlier on by the
default scope. However in the case of an association, we surely don't
want *all* conditions to be removed, otherwise the above would just
become "SELECT * FROM comments" with no foreign key constraint.

To make the above work, we'd have to somehow tag the relation values
which get added when evaluating the default scope in order to
differentiate them from other relation values. Which is way too much
complexity and therefore not worth it when most use cases can be
satisfied with unscope.

Closes #10643, #11061.
Expand double-negative in String#blank? regex