-Adding AddEnumValuesToHelpText option to HelpText.cs

When property AddEnumValuesToHelpText of HelpText is true, the help text generated includes a list of the valid  values of a property when its type is an enum.

The unit tests HelpText.Create_instance_with_enum_options_enabled() and HelpText.Create_instance_with_enum_options_disabled show how to use the AddEnumValuesToHelpText option.

The default value of AddEnumValuesToHelpText is false to avoid problems in existing code that uses HelpText class.