Explicitly indicate that nexttowardl() is identical to nextafterl().

Requested by kamil.

lib/libm/man/nextafter.3

@@ -1,4 +1,4 @@
-.\" $NetBSD: nextafter.3,v 1.7 2019/04/28 07:33:02 wiz Exp $
+.\" $NetBSD: nextafter.3,v 1.8 2019/05/02 15:08:35 mgorny Exp $
 .\"
 .\" Copyright (c) 2011 Jukka Ruohonen <jruohonen@iki.fi>
 .\" All rights reserved.
@@ -78,10 +78,9 @@ The three functions differ only in the type of the return value and
 .Fa x .
 .Pp
 The
-.Fn nexttoward ,
-.Fn nexttowardf
+.Fn nexttoward
 and
-.Fn nexttowardl
+.Fn nexttowardf
 functions are equivalent to the
 .Fn nextafter
 family of functions with two exceptions:
@@ -97,6 +96,10 @@ converted to the type of the function, provided that
 equals
 .Fa y .
 .El
+.Pp
+.Fn nexttowardl
+is equivalent to
+.Fn nextafterl .
 .Sh RETURN VALUES
 Upon successful completion, the described functions return
 the next representable floating-point value as described above.