document filter options user, group and chroot

[Kirill Miazine]

Hi, list

As a new user I have to spend a while with docs, so while reading I 
noticed that user, group and chroot arguments for the filter option were 
not documented, so here's a try.

user and group are even used in the DKIM signing example in smtpd.conf.

Greetings
Kirilldiff --git a/usr.sbin/smtpd/smtpd.conf.5 b/usr.sbin/smtpd/smtpd.conf.5
index a4324e94..e8a93f1b 100644
--- a/usr.sbin/smtpd/smtpd.conf.5
+++ b/usr.sbin/smtpd/smtpd.conf.5
@@ -391,7 +391,8 @@ filter
 backed by the
 .Ar proc-name
 process.
-.It Ic filter Ar filter-name Ic proc-exec Ar command
+.It Ic filter Ar filter-name Ic proc-exec Ar command Ic user Ar username Ic group Ar groupname \
+Ic chroot Ar pathname
 Register and execute
 .Qq proc
 filter
@@ -401,6 +402,13 @@ from
 conformant with the
 .Xr smtpd-filters 7
 API.
+.Ar user
+and
+.Ar group
+options may be used to specify that the command should be run as a specific user or group.
+.Ar chroot
+may be used to limit the command using
+.Xr chroot 2 .
 If
 .Ar command
 starts with a slash it is executed with an absolute path,