Update documentation on zero_buffers / scramble_buffers

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.1 b/fio.1
index 2dee56b..20f0c09 100644
--- a/fio.1
+++ b/fio.1
@@ -430,6 +430,8 @@
 .TP
 .B zero_buffers
 Initialise buffers with all zeros. Default: fill buffers with random data.
+The resulting IO buffers will not be completely zeroed, unless
+\fPscramble_buffers\fR is also turned off.
 .TP
 .B refill_buffers
 If this option is given, fio will refill the IO buffers on every submit. The