Mention that '_' is the same as '.'.

Noted missing by Aleksey Cheusov in PR 41148.

pkgtools/pkg_install/files/info/pkg_info.1

@@ -1,4 +1,4 @@
-.\" $NetBSD: pkg_info.1,v 1.29 2010/02/24 22:07:28 wiz Exp $
+.\" $NetBSD: pkg_info.1,v 1.30 2010/02/27 21:02:01 wiz Exp $
 .\"
 .\" FreeBSD install - a package for the installation and maintenance
 .\" of non-core utilities.
@@ -17,7 +17,7 @@
 .\"
 .\"     @(#)pkg_info.1
 .\"
-.Dd January 22, 2010
+.Dd February 27, 2010
 .Dt PKG_INFO 1
 .Os
 .Sh NAME
@@ -289,7 +289,9 @@ The magic string
 .Dq pl
 equates to a
 .Pa patch level
-and has the same value as a dot in the dewey-decimal ordering schemes.
+and has the same value as a dot in the dewey-decimal ordering schemes,
+as does the underscore
+.Sq _ .
 .Sh ENVIRONMENT
 See
 .Xr pkg_install.conf 5