Improve documentation for BUILDID.

doc/BUILDING.mdoc

@@ -1,4 +1,4 @@
-.\"	$NetBSD: BUILDING.mdoc,v 1.99 2014/05/21 13:11:25 wiz Exp $
+.\"	$NetBSD: BUILDING.mdoc,v 1.100 2014/06/14 12:20:17 apb Exp $
 .\"
 .\" Copyright (c) 2001-2011 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -33,7 +33,7 @@
 .\" Toolchain prefix for commands
 .ds toolprefix nb
 .
-.Dd November 8, 2012
+.Dd June 14, 2014
 .Dt BUILDING 8
 .Os NetBSD
 .
@@ -339,11 +339,17 @@ configuration file specified by
 .
 .It Sy BUILDID
 Identifier for the build.
+If set, this should be a short string that is suitable for use as
+part of a file or directory name.
 The identifier will be appended to
 object directory names, and can be consulted in the
 .Xr make 1
 configuration file in order to set additional build parameters,
 such as compiler flags.
+It will also be used as part of the kernel version string,
+which can be printed by
+.Dq Li uname \-v .
+.DFLTu
 .
 .It Sy BUILDSEED
 GCC uses random numbers when compiling C++ code.