git-rebase.txt: --interactive does not work with --whitespace

Signed-off-by: NMark Burton <markb@ordern.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -246,6 +246,7 @@ OPTIONS
 --whitespace=<nowarn|warn|error|error-all|strip>::
 	This flag is passed to the 'git-apply' program
 	(see linkgit:git-apply[1]) that applies the patch.
+	Incompatible with the --interactive option.
 
 -i::
 --interactive::