More stuff spotted by jnemeth: fix wording for /var/mail, and document

utmpx/wtmpx. While here also document dmesg.boot.

share/man/man7/hier.7

@@ -1,4 +1,4 @@
-.\"	$NetBSD: hier.7,v 1.119 2016/08/16 01:46:51 dholland Exp $
+.\"	$NetBSD: hier.7,v 1.120 2016/08/16 02:29:43 dholland Exp $
 .\"
 .\" Copyright (c) 1990, 1993, 1994
 .\"	The Regents of the University of California.  All rights reserved.
@@ -1177,10 +1177,13 @@ script.
 Login and logout log;
 see
 .Xr utmp 5 .
+.It Sy wtmpx
+Another login and logout log; see
+.Xr utmpx 5 .
 .El
 .Pp
 .It Sy mail/
-User system mailboxes.
+User e-mail inboxes.
 .It Sy msgs/
 System messages; see
 .Xr msgs 1 .
@@ -1197,9 +1200,16 @@ Filesystem quota information.
 System information files, rebuilt after each reboot.
 .Pp
 .Bl -tag -width "utmp" -compact
+.It Sy dmesg.boot
+A dump from
+.Xr dmesg 8
+taken at boot time.
 .It Sy utmp
 Database of currently logged in users; see
 .Xr utmp 5 .
+.It Sy utmpx
+Another database of currently logged in users; see
+.Xr utmpx 5 .
 .El
 .Pp
 .It Sy rwho/