doc/git-log: drop "-r" diff option

This has been the default since 170c04383b (Porcelain level "log" family
should recurse when diffing., 2007-08-27). There's not even a way to
turn it off, so you'd never even want "-r" to override that.

It's not the default for plumbing like diff-tree, of course, but the
option is documented separately there.

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

Documentation/git-log.txt

@@ -146,11 +146,8 @@ below can be used to show the changes made by each commit.
 	in that case, the output represents the changes the merge
 	brought _into_ the then-current branch.
 
--r::
-	Show recursive diffs.
-
 -t::
-	Show the tree objects in the diff output. This implies `-r`.
+	Show the tree objects in the diff output.
 
 :git-log: 1
 include::diff-options.txt[]