Remove a duplicated section [ci skip]

Attachments were previously covered so remove the useless part. Just
move a note about multipart headers set when the mail method is
triggered to the kept section.
Added clarification for destroy link_to description in 5.14 [ci skip]
Typo in Active Record Migrations Guide [ci skip]
Clean the code a bit
Merge pull request #11190 from dkaplan88/typo

Typo in Active Record Migrations Guide
Merge pull request #11189 from jetthoughts/destroy_link_description_for_getting_started

Added clarification for destroy link_to description in 5.14
Merge pull request #11180 from robin850/patch-5

Remove a duplicated section [ci skip]
Synchronize PostController#create code from 5.6 to others sections. [ci skip]

Fixed `permit` using for comments.
don't shadow `through_scope` method name with local var.
Merge pull request #11186 from jetthoughts/synchronize_create_method_body

Synchronize PostController#create code from 5.6 to others sections in Getting Started guide.
Add `#any_of` query method to active_record

This method is inspired by [any_of from
mongoid](http://two.mongoid.org/docs/querying/criteria.html#any_of).

It allows to compute an `OR` like query that leverages AR's `#where` syntax:

    manual_removal = User.where(id: params[:users][:destroy_ids])
    users = User.any_of(manual_removal, "email like '%@example.com'", {banned: true}).destroy_all

It can be used directly on model class, or through an association, or
behind an other relation.

    manual_removal = User.where(id: params[:users][:destroy_ids])
    User.any_of(manual_removal, "email like '%@example.com'", {banned: true})
    @company.users.any_of(manual_removal, "email like '%@example.com'", {banned: true})
    User.where(offline: false).any_of( manual_removal, "email like '%@example.com'", {banned: true})

Its main purpose is to both :

* remove the need to write a sql string when we want an `OR`
* allow to write dynamic `OR` queries, which would be a pain with a string

There's also a negative version, `#none_of`. This will return all active
users :

    banned_users = User.where(banned: true)
    unconfirmed_users = User.where("confirmed_at IS NULL")
    active_users = User.none_of(banned_users, unconfirmed_users)