Document the fact that Linux binaries depend on pty(4).

The source part of the review will be addressed in a different way.

Reviewed by:	emaste, brooks
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D21492

share/man/man4/linux.4

@@ -139,6 +139,7 @@ limited Linux system file system
 .El
 .Sh SEE ALSO
 .Xr brandelf 1 ,
+.Xr pty 4 ,
 .Xr elf 5 ,
 .Xr fdescfs 5 ,
 .Xr linprocfs 5 ,

share/man/man4/pty.4

@@ -27,19 +27,21 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 20, 2008
+.Dd October 28, 2019
 .Dt PTY 4
 .Os
 .Sh NAME
 .Nm pty
-.Nd BSD-style and System V-style compatibility pseudo-terminal driver
+.Nd old-style compatibility pseudo-terminal driver
 .Sh SYNOPSIS
 .Cd "device pty"
 .Sh DESCRIPTION
 The
 .Nm
 driver provides support for the traditional BSD naming scheme that was
-used for accessing pseudo-terminals.
+used for accessing pseudo-terminals before it was replaced by
+.Xr pts 4 .
+This traditional naming is still used in Linux.
 When the device
 .Pa /dev/ptyXX
 is being opened, a new terminal shall be created with the
@@ -59,7 +61,8 @@ interface.
 It is only provided for compatibility with older C libraries
 that tried to open such devices when
 .Xr posix_openpt 2
-was being called.
+was being called,
+and for running Linux binaries.
 .Sh FILES
 The BSD-style compatibility pseudo-terminal driver uses the following
 device names: