Added purpose and description.

tools/quartus/set_quartus

@@ -20,6 +20,11 @@
 #
 ###############################################################################
 
+# Purpose: Map command line argument to a tool version
+# Description:
+#   By using command line arguments like "", "unb1", "unb2" to select the tool
+#   version it is easier to manage combinations of tool versions.
+
 # General tool and project settings
 # - use '. <script>.sh' to have the settings apply in this shell, otherwise they get lost when <script>.sh returns