Documentation/log: clarify what --raw means

Documentation/log: clarify what --raw means

There are several "raw formats", and describing --raw as "Generate the
raw format" in the documentation for git-log seems to imply that it
generates the raw *log* format.

Clarify the wording by saying "raw diff format" explicitly, and make a
special-case for "git log": "git log --raw" does not just change the
format, it shows something which is not shown by default.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>