Related to f1af967e. [ci skip]

Thanks for patch @sgrif :trollface:

Closes #18216

In the AR Associations Guide, this PR fixes:

- The counter_cache declaration is now shown only in the `belongs_to`
association.

- The docs stated that the counter_cache declaration needs to be on the
`has_many` side; now corrected to the `belongs_to` side.

- Split the custom column explanation out to a separate paragraph.

- Simplify the NOTE because it is true both with and without a custom
column name.

Deprecate `required` option in favor of `optional` for belongs_to.

- Changed `IN` to `ON` in all note sentences in guides.

References #18148.

You need to specify the counter_cache option on the has_many side of the association when using a custom counter cache column. This is documented on the has_many association here: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many [ci skip]

fixes issue #16698

Follow up of #16722.

https://github.com/rails/rails/pull/16722

also normalize all instance gained methods’ sentence.

missed one migration script in last commit

remove some empty lines

using the belongs_to index option to be more concise

Updated association_basics.md to include how to write migrations for self joins

Update association_basics.md

Corrected the update

Update association_basics.md

Typo fix

Changed :employee to :manager

removed unnecessary test case and improved test case for belongs_to having invalid  options

Also clean up some trailing spaces on line 1143-1146.

Based on the instructions further down in section 3.3.2.

[ci skip]

Added comment to a guide about necessity of `:dependent` option for associations with NOT NULL constraint

Done the following changes:
 1. Replace uniq call in example with autosave as uniq is not supported
 on has_and_belongs_to_many
 2. Add :readonly option to list of supported call as per
 [EdgeApi](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many)