Update documentation for collection<< to reflect << on new records.

collection<< does not automatically fire update sql unless the parent object
is not a new record (and thus has a primary key).

eg:

p = Post.new
p.comments << Comment.new

does not call update sql until:

p.save

at which point it saves the parent object and then the collection object.

changes that to the
Merge pull request #9630 from robertjwhitney/update_collection_docs

Update documentation for collection<< to reflect behavior for new parent objects.