document the special packaging parameters, and remove the obsolete uses

BuildParams

@@ -48,13 +48,24 @@ Linux.cxx_version = /usr/bin/g++-4.6
 ################################################################
 viewer_channel = "Second Life Test"
 
-# Setup default packaging parameters.
+################################################################
+# Special packaging parameters.
+# These parameters can be used to create additional packages
+# which identify themselves in a distinct way with either
+# a sourceid (sent to web services) or a channel name (sent to login)
+# the default sourceid should always be a null string:
 sourceid = ""
-additional_packages = "Amazon Desura"
-Amazon_sourceid = "1207v_Amazon"
-Amazon_viewer_channel_suffix = "Amazon"
-Desura_sourceid = "1208_desura"
-Desura_viewer_channel_suffix = "Desura"
+# the additional_packages variable is a blank separated list of package prefixes:
+additional_packages = ""
+# to set the special values for a package, create variables using each prefix:
+#   additional_packages = "Foo Bar"
+#   Foo_sourceid = "bingo"
+#   Foo_viewer_channel_suffix = "Foo"
+#   Bar_sourceid = "bongo"
+#   Bar_viewer_channel_suffix = "Bar"
+# the viewer_channel_suffix is prefixed by a blank and then appended to the viewer_channel
+# for the package in a setting that overrides the compiled-in value
+################################################################
 
 # Notifications - to configure email notices, add a setting like this:
 # <username>_<reponame>.email = <email-address>