Document that using 70 as size doesn't work in wxFont ctor

Mention the workaround of using SetPointSize(70) instead.

See #12315.

interface/wx/font.h

@@ -485,7 +485,11 @@ public:
         @endcode
 
         @param pointSize
-            Size in points. See SetPointSize() for more info.
+            Size in points. See SetPointSize() for more info. Notice that, for
+            historical reasons, the value 70 here is interpreted at @c
+            wxDEFAULT and results in creation of the font with the default size
+            and not of a font with the size of 70pt. If you really need the
+            latter, please use SetPointSize(70).
         @param family
             The font family: a generic portable way of referring to fonts without specifying a
             facename. This parameter must be one of the ::wxFontFamily enumeration values.