improving inline documentation

CHANGELOG.rdoc

lib/cancan/ability.rb

lib/cancan/active_record_additions.rb

lib/cancan/can_definition.rb

lib/cancan/controller_additions.rb

lib/cancan/controller_resource.rb

lib/cancan/exceptions.rb

lib/cancan/resource_authorization.rb

spec/cancan/ability_spec.rb

spec/cancan/active_record_additions_spec.rb

spec/cancan/can_definition_spec.rb

spec/cancan/controller_additions_spec.rb

spec/cancan/controller_resource_spec.rb

spec/cancan/exceptions_spec.rb

spec/cancan/resource_authorization_spec.rb

spec/spec_helper.rb