Update cpus_allowed_policy=split documentation

Instead of erroring out on fewer CPUs than threads/processes,
fio now does roundrobin over the set.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.1 b/fio.1
index 863b0e0..764e323 100644
--- a/fio.1
+++ b/fio.1
@@ -847,8 +847,9 @@
 .RE
 .P
 \fBshared\fR is the default behaviour, if the option isn't specified. If
-\fBsplit\fR is specified, then fio will error out if there are more jobs
-defined than CPUs given in the set.
+\fBsplit\fR is specified, then fio will assign one cpu per job. If not enough
+CPUs are given for the jobs listed, then fio will roundrobin the CPUs in
+the set.
 .RE
 .P
 .TP