Clarify and extend the "git diff" format documentation

Clarify and extend the "git diff" format documentation

Move the similarity and dissimilarity index header description closer to
where those extended headers are described.

Describe and/or clarify the format used for file modes, pathnames, and
the index header.

Document that all "old" files refer to the state before applying the
*entire* output, and all "new" files refer to the state thereafter.

Signed-off-by: Andreas Gruenbacher <redacted>
Signed-off-by: Junio C Hamano <redacted>