Minor doc fixes for wxSizerItem methods

Fix typo in GetRatio() documentation and add documentation for
SetInitSize().

Closes https://github.com/wxWidgets/wxWidgets/pull/1882

interface/wx/sizer.h

@@ -1179,7 +1179,7 @@ public:
     int GetProportion() const;
 
     /**
-        Get the ration item attribute.
+        Get the ratio item attribute.
     */
     float GetRatio() const;
 
@@ -1264,7 +1264,10 @@ public:
     void SetId(int id);
 
     /**
-        @todo docme.
+        Sets the minimum size to be allocated for this item.
+
+        If this item is a window, the @a size is also passed to
+        wxWindow::SetMinSize().
     */
     void SetInitSize(int x, int y);