Clarify wxEVT_SLIDER event handler argument type in the documentation.

Unlike all other events generated by wxSlider, this one receives a
wxCommandEvent and not a wxScrollEvent.

Unfortunately @beginEventEmissionTable doesn't handle the case of a class
producing events of different types, perhaps it shouldn't be used at all here.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76719 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775