echo.1: add a note about other echo implementations

The other BSDs all have a note reminding that many shells have their
own internal echo implementations which may vary from this utility, so
add one. (Much of the wording is borrowed from FreeBSD's man page.)

(The other BSDs also have notes about the -n option not really being
portable, and printf[1] being preferable, we might want to add
something about that, too.)

bin/echo/echo.1

@@ -1,4 +1,4 @@
-.\"	$NetBSD: echo.1,v 1.17 2017/07/04 06:48:41 wiz Exp $
+.\"	$NetBSD: echo.1,v 1.18 2020/02/26 02:09:46 gutteridge Exp $
 .\"
 .\" Copyright (c) 1990, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -32,7 +32,7 @@
 .\"
 .\"	@(#)echo.1	8.1 (Berkeley) 7/22/93
 .\"
-.Dd August 14, 2016
+.Dd February 26, 2020
 .Dt ECHO 1
 .Os
 .Sh NAME
@@ -54,9 +54,20 @@ The following option is available:
 .It Fl n
 Do not print the trailing newline character.
 .El
+.Pp
+Note that many shells (e.g.,
+.Xr csh 1
+and
+.Xr ksh 1 )
+provide a built-in
+.Nm
+command which tends to differ from this utility in the treatment of options
+and backslashes.
 .Sh EXIT STATUS
 .Ex -std echo
 .Sh SEE ALSO
+.Xr csh 1
+.Xr ksh 1
 .Xr printf 1
 .Sh STANDARDS
 The