Document that wxDC::DrawSpline() requires at least 3 points

This is the case for wxMSW implementation and while the other ones seem
to accept 1 or 2 points too, document the most stringent requirement,
especially because not satisfying it results not "just" in assertion
failure, but also in a crash in wxMSW.

This should obviously be improved further by, at the very least, using
wxCHECK and not wxASSERT in wxMSW code or, maybe, actually handling the
degenerate cases there if all the other ports really support them.

See #19172.

interface/wx/dc.h

@@ -714,6 +714,8 @@ public:
     /**
         Draws a spline between all given points using the current pen.
 
+        The number of points must be at least 3 for the spline to be drawn.
+
         @beginWxPerlOnly
         Not supported by wxPerl.
         @endWxPerlOnly