Document that wxUSE_NANOSVG is available starting in 3.1.7 only

Don't bother repeating it for wxUSE_NANOSVG_EXTERNAL as it should be
clear enough that this option doesn't make sense without wxUSE_NANOSVG
anyhow.

docs/doxygen/mainpages/const_wxusedef.h

@@ -177,7 +177,7 @@ library:
 @itemdef{wxUSE_MINIFRAME, Use wxMiniFrame class.}
 @itemdef{wxUSE_MOUSEWHEEL, Support mouse wheel events.}
 @itemdef{wxUSE_MSGDLG, Use wxMessageDialog class and wxMessageBox function.}
-@itemdef{wxUSE_NANOSVG, Use NanoSVG for rasterizing SVG.}
+@itemdef{wxUSE_NANOSVG, Use NanoSVG for rasterizing SVG (this option is new since wxWidgets 3.1.7).}
 @itemdef{wxUSE_NANOSVG_EXTERNAL, Use an external NanoSVG library when wxUSE_NANOSVG is enabled.}
 @itemdef{wxUSE_NANOSVG_EXTERNAL_ENABLE_IMPL, When the external NanoSVG library is header only, define this to enable its implementation.}
 @itemdef{wxUSE_NATIVE_DATAVIEWCTRL, Use native wxDataViewCtrl class (this option is new since wxWidgets 3.1.4).}