Some databases like MySQL allow defining collation charset for specific
columns.

Move comment about microseconds [ci skip]

gem 'mysql2', '~> 0.3.18'

Follow up #18914.

`:nodoc:` postgresql's change_column

Its nodoc'ed for the other implementations, and doc'ed in the base
class, just like the other change_column* methods.

The microseconds handling was already moved to `Quoting#quoted_date`.

[ci skip] Uniquiness with scope can have one or more arguments

:nodoc: change_column_null in the implmenting adapters

Remove unused require

`change_column_null` is doc'ed only in
ActiveRecord::ConnectionAdapters::SchemaStatements, so it would make
sense to :nodoc: it elsewhere.

`setup do` creates unnecessary allocations of proc objects in test
callbacks. This prevents that from happening and results in faster code.

Originally I had done this as `def setup` and all Railties tests passed.
See 044f9ab7. Later it was reported there was an issue with this that
caused routes in tests to be nil because devs don't generally call
`super` in their test setups. Because of that I reverted the commit
until I could find a suitble replacement.

`before_setup` esentially does the same thing but without the
requirement that applications call `super` in their test setups.

`*args` is not required here and should be avoided when not necessary
because `*args` are slower than `args` and create unnecessary array
allocations.

Clean up debugging guide and fix minor grammatical errors[ci skip]

[ci skip] Fix example codes of form_for method

Added missing fullstop in engine API docs [ci skip]

Improvement on Activerecord CHANGELOG.md [ci skip]

Update documentation to contain a 24 char token [ci skip]

This illustrates the purpose of the block for `link_to_if` and
`link_to_unless` helper methods.

It should help to prevent further mistakes like #19844.

This reverts commit d459b001, reversing
changes made to 4d4950fa.

😓

The block is not supposed to be passed to `link_to`. It's used for a
customized behavior of the `condtion = false` case. The docs
illustrate that like so:

```
<%=
   link_to_if(@current_user.nil?, "Login", { controller: "sessions", action: "new" }) do
     link_to(@current_user.login, { controller: "accounts", action: "show", id: @current_user })
   end
%>
```

Update url_helper.rb

match a expected value with message of `assert_equal` in AJ helper methods

 Nodoc validate_index_length! method

 Added documentation for PostGreSQL database_statements [ci skip]

 - This method is used only by adapters to validate length of new index names.

 - Added documentation for index_name_exists? and rename_index.
 - Also changed rails to \Rails in documentation of
   allowed_index_name_length.

Fix style/grammar/typos in Caching guide.

* avoid using unnecessary commas
* fix some typos