git-notes(1): add a section about the meaning of history

git-notes(1): add a section about the meaning of history

To the displaying code, the only interesting thing about a notes ref
is that it has a tree of the required format.  However, notes actually
have a history since they are recorded as successive commits.

Make a note about the existence of this history in the manpage, but
keep some doors open if we want to change the details.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>