The -F flag of swapon(8) requires -a to work.

Before this change, swapon(8) implied that -F works as a standalone option,
which is not the case and would produce a usage message. This change extends
the description of the -F option to mention that -a is required with it.

PR:		238551
Submitted by:	Christian Baltini
MFC after:	5 days

sbin/swapon/swapon.8

@@ -28,7 +28,7 @@
 .\"     @(#)swapon.8	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd June 21, 2019
+.Dd May 19, 2020
 .Dt SWAPON 8
 .Os
 .Sh NAME
@@ -98,7 +98,7 @@ To delay
 .Nm swapon
 for a device until after
 .Nm savecore
-has copied the crash dump to another location, use the 
+has copied the crash dump to another location, use the
 .Dq late
 option.
 .Pp
@@ -143,7 +143,8 @@ are as follows:
 .It Fl F Ar fstab
 Specify the
 .Pa fstab
-file to use.
+file to use (in conjunction with
+.Fl a Ns ).
 .El
 .Pp
 The