Documentation: Generalize git-notes docs to 'objects' instead of 'commits'

Documentation: Generalize git-notes docs to 'objects' instead of 'commits'

Notes can annotate arbitrary objects (not only commits), but this is not
reflected in the current documentation.

This patch rewrites the git-notes documentation to talk about 'objects'
instead of 'commits'. However, the discussion on commit notes and how
they are displayed by 'git log' is largely preserved.

Finally, I add myself to the Author/Documentation credits, since most of
the lines in the git-notes code and docs are blamed on me.

Cc: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>