Adding documentation to the Rails initialization guide. [ci skip]
Merge pull request #11160 from wangjohn/documentation_for_railties_initialization_guide

Adding documentation to the Rails initialization guide.
writing the new body can cause the response to be committed and the
request thread to return up the stack before the instance variable is
assigned.  Synchronize so that the ivar is assigned before the other
thread can activate.

fixes #10984
use bytesize rather than force encoding
remove deprecated implicit join references.
fetch value(s) from stringified options
Update a broken link into the fixtures' template
Merge pull request #11175 from robin850/patch-4

Update a broken link into the fixtures' template [ci skip]
Merge pull request #11152 from senny/remove_implicit_join_references

remove deprecated implicit join references
provide a more sementicthe local variables name for ActiveModel::Validations::Clusivity#include? method
the original name `exclusion` is a bit confusing when using with the method `inclusion_method`
rename it to a more logic neutral name.
Merge pull request #11156 from JonRowe/fix_tags_picking_string_keys_from_non_stringified_options

Fetch value(s) from stringified options in tags that stringify options
Merge pull request #11177 from yangchenyun/mine

provide a more sementic local variables name in ActiveModel::Validation::Clusivity
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]
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