doc/git-log: clarify handling of merge commit diffs

It can be surprising that git-log doesn't show any diff for merge
commits by default. Arguably "--cc" would be a reasonable default, but
it's very expensive (which is why we turn it on for "git show" but not
for "git log"). Let's at least document the current behavior, including
the recent "--first-parent implies -m" case

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

Documentation/git-log.txt

@@ -117,6 +117,13 @@ DIFF FORMATTING
 By default, `git log` does not generate any diff output. The options
 below can be used to show the changes made by each commit.
 
+Note that unless one of `-c`, `--cc`, or `-m` is given, merge commits
+will never show a diff, even if a diff format like `--patch` is
+selected, nor will they match search options like `-S`. The exception is
+when `--first-parent` is in use, in which merges are treated like normal
+single-parent commits (this can be overridden by providing a
+combined-diff option or with `--no-diff-merges`).
+
 -c::
 	With this option, diff output for a merge commit
 	shows the differences from each of the parents to the merge result