Updated DOCS for engines and added a couple TODOs. Also, commented internal railties rake tasks description.

Rename rake railties:copy_migrations to rake railties:install:migrations and fix it to work with new copying strategy

The purpose of this change is to allow copying fail on the same names.
Migrations change database and they should be treated with caution,
if 2 migrations are named the same it's much better to skip migration
and allow user decide if it should be copied or not.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

AssociationCollection#include? working properly for objects added with build method [#3472 state:resolved]

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

With config.generators becomes a way to configure generators
for current instance only. For example:

module Blog
  class Engine < Rails::Engine
    config.generators do |g|
      g.orm :active_record
    end

    config.app_generators do |g|
      g.test_framework :rspec
    end
  end
end

such definition sets :active_record as orm for engine and :rspec
as test_framework for application. The values set with app_generators
can be overwritten in application using config.generators as you would
normally do:

module MyApp
  class Application < Rails::Application
    config.generators do |g|
      g.test_framework :test_unit
    end
  end
end