fcntl.2: clarify arg use for F_SETFD

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential revision:	https://reviews.freebsd.org/D57162

Author: kostikbel

lib/libsys/fcntl.2

@@ -176,7 +176,9 @@ descriptor to also have the flag set.
 .El
 .It Dv F_SETFD
 Set flags associated with
-.Fa fd .
+.Fa fd
+to
+.Fa arg .
 The available flags are
 .Dv FD_CLOEXEC ,
 .Dv FD_CLOFORK