nlist.3: Add discouraged use notice

It is a relic from a.out days and is poorly specified. Although ELF
support was added to nlist, there are better ways to access ELF data.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57078

lib/libc/gen/nlist.3

@@ -40,6 +40,10 @@
 .Sh DESCRIPTION
 The
 .Fn nlist
+function is provided for compatibility with legacy applications.
+Its use is discouraged.
+The
+.Fn nlist
 function
 retrieves name list entries from the
 .Xr elf 5