Document wxDC::{Set,Get}Logical{Scale,Origin}() functions.

They are relatively important, especially the origin-related ones as
SetUserScale() can (should?) be used instead of SetLogicalScale(), but
SetLogicalOrigin() can be more convenient than SetDeviceOrigin(), so provide
at least skeletal documentation for them.

(cherry picked from commit 394b04b7e4)

interface/wx/dc.h

@@ -1590,12 +1590,36 @@ public:
     wxBitmap GetAsBitmap(const wxRect *subrect = NULL) const;
 
 
+    /**
+        Set the scale to use for translating wxDC coordinates to the physical
+        pixels.
+
+        The effect of calling this function is similar to that of calling
+        SetUserScale().
+     */
     void SetLogicalScale(double x, double y);
+
+    /**
+        Return the scale set by the last call to SetLogicalScale().
+     */
     void GetLogicalScale(double *x, double *y) const;
+
+    /**
+        Change the offset used for translating wxDC coordinates.
+
+        @see SetLogicalOrigin(), SetDeviceOrigin()
+     */
     void SetLogicalOrigin(wxCoord x, wxCoord y);
+
+    //@{
+    /**
+        Return the coordinates of the logical point (0, 0).
+
+        @see SetLogicalOrigin()
+     */
     void GetLogicalOrigin(wxCoord *x, wxCoord *y) const;
     wxPoint GetLogicalOrigin() const;
-    
+    //@}
 };