3-2-stable documentation: the possibility to use a hash of hashes for nested attributes

Document the possibility to use a hash of hashes for nested attributes for a one-to-many association (in addition to the documented possibility to use an array of hashes).

Align indentation in comments.

This is for 3-2-stable.  I have already pushed it to docrails master branch, but i do not know if i can update documentation in 3-2-stable myself i haven't found how.

This only updates comments.

I have just spent some time figuring out that nested attributes work with a hash of hashes too.