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

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