ps(1): Add a standard exit status section

Reviewed by:	bcr, eadler
Approved by:	krion (mentor)
MFC after:      3 days
Differential Revision:	https://reviews.freebsd.org/D17146

bin/ps/ps.1

@@ -29,7 +29,7 @@
 .\"     @(#)ps.1	8.3 (Berkeley) 4/18/94
 .\" $FreeBSD$
 .\"
-.Dd March 13, 2018
+.Dd October 31, 2018
 .Dt PS 1
 .Os
 .Sh NAME
@@ -750,6 +750,8 @@ attempts to automatically determine the terminal width.
 .It Pa /boot/kernel/kernel
 default system namelist
 .El
+.Sh EXIT STATUS
+.Ex -std
 .Sh EXAMPLES
 Display information on all system processes:
 .Pp