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]
store a symbol directly inside DISPATCH_CACHE for extra roflscale
Clean the code a bit
updated AS:Callbacks doc for terminator option in define_callbacks method
The change is commited at ba552764344bc0a3c25b8576ec11f127ceaa16da
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.
remove evals from AM::Validations::Callbacks
follow the same refactor at a63a964a5d1ed02cf0df1b1a33a96ed2a9fa987b
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.
Merge pull request #11196 from yangchenyun/remove_evals_in_am_callbacks

Remove evals from AM::Validations::Callbacks
fix typo in ActiveModel::Error docs [ci skip]
Merge pull request #11192 from charliesome/extra-roflscale

Store a symbol directly inside DISPATCH_CACHE for additional roflscale
Adding find_by in guide to methods that trigger after_find
unified the param names across all callbacks manipulation methods
_ Rename the define_callbacks params to `names`
  - in order to match the naming conventions for `get_callbacks` and `set_callbacks` at https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L736-743
  - `define_callbacks` just register names(events), not define the real callback functions.
- Rename the `reset_callbacks` params
Using preferred find_by syntax in guides
Merge pull request #11204 from Gawyn/find_by-triggers-after_find

Adding find_by in guide to methods that trigger after_find