docs: clarify meaning of -M for git-log

As an option to the "diff" family, it is fairly obvious what
"detect renames" means. However, for revision traversal, the
"-M" option is just included in the long list of options,
with no indication that it is about showing renames in diffs
versus following renames. Let's make it more explicit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -177,7 +177,14 @@ endif::git-format-patch[]
 	Break complete rewrite changes into pairs of delete and create.
 
 -M::
+ifndef::git-log[]
 	Detect renames.
+endif::git-log[]
+ifdef::git-log[]
+	If generating diffs, detect and report renames for each commit.
+	For following files across renames while traversing history, see
+	`--follow`.
+endif::git-log[]
 
 -C::
 	Detect copies as well as renames.  See also `--find-copies-harder`.