Add verify_only to man page

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.1 b/fio.1
index b5ff80c..3cdfd27 100644
--- a/fio.1
+++ b/fio.1
@@ -934,6 +934,13 @@
 Specifies the number of iterations (runs of the same workload) of this job.
 Default: 1.
 .TP
+.BI verify_only \fR=\fPbool
+Do not perform the specified workload, only verify data still matches previous
+invocation of this workload. This option allows one to check data multiple
+times at a later date without overwriting it. This option makes sense only for
+workloads that write data, and does not support workloads with the
+\fBtime_based\fR option set.
+.TP
 .BI do_verify \fR=\fPbool
 Run the verify phase after a write phase.  Only valid if \fBverify\fR is set.
 Default: true.