DetailedHelpTextAttribute

As the name of this attribute indicates, you can specify the detailed help text for an argument.
This text is displayed when the help command for this parameter is invoked explicitly.

In case of an argument or an option

XCopyApplication.exe -wait -?

In case of a command parameter

XCopyApplication.exe -copy -destinationfile -?

This would look like

NOTE
If no DetailedHelpTextAttribute is specified for a parameter, the normal help text gets displayed.