Fix file name in document [ci skip]

This file was renamed in c8a25386d.
Fixing the examples for button_to

The examples do not generate the output they intend to due to not explicitly declaring the hash separations. This causes it to be treated as one parameter instead of the intended two parameters.
Change Class#subclasses doc example
Add ActiveRecord.count documentation when used on group relations
Add documentation for ActiveRecord.select when use strings as arguments
Add example for ActiveRecord::Base.group using a string as argument
Add example for ActiveRecord::Base.joins using a string as argument
Validations guide: validates_with init clarification.

As discussed with @josevalim on Ruby Rogues Parley.
Clarify ActiveModel::Validator initialization in docs.

As discussed with @josevalim on Ruby Rogues Parley.
Fix CoffeeScript example

`a[data-color]` is not used in example html.
Fix supported ruby version for Rails 4
Fix typo.
[ci skip]
Detail Ruby 1.8 ships with TestUnit and Ruby 1.9 with MiniTest
Correct bullet content line wrapping

The `:prefix` bullet content was not properly wrapping,
dropping to the following line because the additional
content was not indented properly. With this change, it
will now display properly the way `:discard_type` does.
incorrect content of models/comment.rb after 'rails generate'
Revert "incorrect content of models/comment.rb after 'rails generate'"

This reverts commit 0f0803fda5df60319d8646bb4f3e71f2e596316a.

Reason: The content was correct since attr_accessible was removed from
rails master.
copy edits [ci skip]
Merge branch 'master' of github.com:lifo/docrails

Conflicts:
	guides/source/upgrading_ruby_on_rails.md