Fix `rails plugin --help`

Right now if you run the `rails plugin --help` command it fails because rails expects a command in `railties/lib/rails/commands/plugin.rb` that does not exist because the file is named `plugin_new`. This is the error:

```
ruby-2.0.0-p0  ~/documents/projects/tmp/vanilla (master)
$ rails plugin --help
/Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/commands/plugin (LoadError)
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0/lib/rails/commands.rb:49:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```
Merge pull request #11176 from schneems/schneems/plugin_new_help_fix

Fix `rails plugin --help`
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.
Merge remote-tracking branch 'upstream/master'