bring index actions back in caching guide, REST conventions are encouraged and surrounding text assumes index

See https://rails.lighthouseapp.com/projects/16213/tickets/23-rails-i18

hasn't.

Also change the assert reference to be more consistent with the rest
of the documentation.

Update ActionMailer guide's '2.3 Complete List of Action Mailer User-Settable Attributes' to show that the attribute 'cc' and 'bcc' can take a string or an Array of strings.

Update ActionMailer guide's 'Sending Multipart Emails with Attachments' section to show explicitly which order text/plain and text/html parts must be in as well as the proper way to define the multipart/alternative part.

Rails numbering: 2.2.2 and 2.3.2 are the release versions, so original wording is fine. Release notes refer to the first *release* version, not the the earlier RC versions.

This reverts commit 858dbeb4.

not the initial release.

It would be good to make this more explicit still.

The motivation for the change was that ActiveRecords find_in_batches
is documented to have been released in rails 2.3, but was actually
only released in 2.3.2.