ParameterGroup description should be listed as optional

[SpeckledFleebeedoo]

The description field for a ParameterGroup in the runtime docs json API is not listed as optional but is not present for a number of them, so it should probably be listed as optional. The first example I ran into was on LuaControl::set_gui_arrow.

Alternatively, for consistency, it may be better if these are given descriptions in the API, as currently the ParameterGroups are the only types in the API that may lack descriptions.