Seems clear enough to me... you can set it to True, False (using the launguage's constants for true or false) or toggle it with WdConstants.WdToggle.
I'm not sure why you need the option to toggle it, though, seeing as toggling would be the same thing as View.ShowXMLMarkup = !(View.ShowXMLMarkup); if View.ShowXMLMarkup were implemented as a bool rather than an int32 (which they did to allow for the WdToggle option). As is, you need a more complicated conditional to toggle... so the WdToggle option becomes useful only because it's present.
