For docs, "Save" just gives you the reassurance that you won't lose your work if the browser/connection/site crashes, yet you haven't published it for all to see if you weren't finished.
For blog posts you can open the Extended Options and choose whether the post will be published, unpublished or scheduled for later publication.
Can't say I know where this would be documented -- this is based on my experience.
Thanks for the comments.
This is not the experience when you have already published the document. Yesterday, one of our Marketing folks was updating a blog with new pictures and hit save, because she wanted to save her changes, but wasn't ready to publish.
It's confusing that the "publish" button is still there on drafts, if it doesn't actually have a different action from "save" on already published blogs/documents.