I don't expect an author's note at all. The author is is under no requirement to communicate with me, the reader, except within the story.
If the author chooses to communicate, I would hope for some indication of where the line may be between fact & fiction. My all time favorite historical novelist, Dorothy Dunnett, does this by indicating in the list of characters at the beginning which ones are real people vs fictional. That's sufficient. If the author chooses to identify whether specific known facts have been altered for the story, it's nice but not at all necessary (unless the change is made to something very well known where people are likely to call you out for sloppy research if you don't flag it, or as angeliz2k says, if you've included something that sounds like it must be fictional but really happened).
Speaking of research, it's also nice to see a bit of bibliography -- not necessarily the sources you depended on the most in writing your story, but works you recommend readers can look at if they want to learn more about the historical setting, or about particularly interesting or noteworthy aspects of it that are relevant to the story. I remember a very informative author's note of this kind that Kate Elliott wrote some years back for a fantasy series whose setting was based very closely on the Holy Roman Empire of the 10th century -- a fascinating period that's not well known to most of us.
Beyond that... well, it's your note, so whatever you want to say to the readers is what goes in. (Though if it gets too chatty and personal-bloggish I, as a reader who doesn't know you personally and doesn't actually have a burning interest in the writer-as-a-person outside of their writing, may sit here rolling my eyes and thinking, okay, get to the point...? Other people like that kind of thing more than I do.)