makeman: add a comment describing purpose and invocation

Reviewed by:	jhb, ngie
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D10389

tools/build/options/makeman

@@ -1,8 +1,22 @@
 #!/bin/sh
 #
 # This file is in the public domain.
-#
 # $FreeBSD$
+#
+# This script is used to create the src.conf.5 man page, using template text
+# contained herein and the contents of the WITH_* and WITHOUT_* files in the
+# same directory. Each WITH_* and WITHOUT_* file documents the effect of the
+# /etc/src.conf knob with the same name.
+#
+# This script invokes "make showconfig" for each supported architecture in
+# order to determine, for each option, whether the default setting is always
+# WITH, always WITHOUT, or is architecture-dependent. It also determines
+# and describes dependencies between options.
+#
+# Usage:
+#
+#     cd tools/build/options
+#     sh makeman > ../../../share/man/man5/src.conf.5
 
 set -o errexit
 export LC_ALL=C
@@ -128,7 +142,7 @@ main()
 
 	fbsdid='$'FreeBSD'$'
 	cat <<EOF
-.\" DO NOT EDIT-- this file is automatically generated.
+.\" DO NOT EDIT-- this file is generated by tools/build/options/makeman.
 .\" ${fbsdid}
 .Dd $(echo $(LC_TIME=C date +'%B %e, %Y'))
 .Dt SRC.CONF 5