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.