swapon: clarify swapon.8 priority option

Value -1 is valid argument for -p, even though it might not make much
sense as the -1 is in use when priority is not specified at all.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

sys-utils/swapon.8

@@ -150,8 +150,8 @@ is needed.)
 .B "\-p, \-\-priority \fIpriority\fP"
 Specify the priority of the swap device.
 .I priority
-is a value between 0 and 32767. Higher numbers indicate higher
-priority. See
+is a value between \-1 and 32767.  Higher numbers indicate
+higher priority.  See
 .BR swapon (2)
 for a full description of swap priorities. Add
 .BI pri= value
@@ -159,6 +159,7 @@ to the option field of
 .I /etc/fstab
 for use with
 .BR "swapon -a" .
+When priority is not defined it defaults to \-1.
 .TP
 .B "\-s, \-\-summary"
 Display swap usage summary by device. Equivalent to "cat /proc/swaps".