The compile time constant limit on number of swap devices was removed in 5.2.

As such, remove the EINVAL error saying so.  Currently the vm.nswapdev sysctl
just represents the number of added swap devices.

MFC after:	1 week

lib/libc/sys/swapon.2

@@ -28,7 +28,7 @@
 .\"     @(#)swapon.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 4, 1993
+.Dd October 4, 2013
 .Dt SWAPON 2
 .Os
 .Sh NAME
@@ -98,10 +98,6 @@ Additionally,
 .Fn swapon
 can fail for the following reasons:
 .Bl -tag -width Er
-.It Bq Er EINVAL
-The system has reached the boot-time limit on the number of
-swap devices,
-.Va vm.nswapdev .
 .It Bq Er ENOTBLK
 The
 .Fa special