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>

Documentation/diff-options.txt

@@ -41,8 +41,17 @@ endif::git-format-patch[]
 
 ifndef::git-format-patch[]
 --raw::
-	Generate the raw format.
+ifndef::git-log[]
+	Generate the diff in raw format.
 	{git-diff-core? This is the default.}
+endif::git-log[]
+ifdef::git-log[]
+	For each commit, show a summary of changes using the raw diff
+	format. See the "RAW OUTPUT FORMAT" section of
+	linkgit:git-diff[1]. This is different from showing the log
+	itself in raw format, which you can achieve with
+	`--format=raw`.
+endif::git-log[]
 endif::git-format-patch[]
 
 ifndef::git-format-patch[]