Fix and improve wxDC background mode documentation

Use wxBRUSHSTYLE_SOLID/TRANSPARENT constants instead of wxPENSTYLE_XXX
ones, as even though they have the same values, using brush style for
the background makes more sense.

Also document that the default background mode is transparent.

interface/wx/dc.h

@@ -997,7 +997,7 @@ class wxDC : public wxObject
     //@{
 
     /**
-        Returns the current background mode: @c wxPENSTYLE_SOLID or @c wxPENSTYLE_TRANSPARENT.
+        Returns the current background mode: @c wxBRUSHSTYLE_SOLID or @c wxBRUSHSTYLE_TRANSPARENT.
 
         @see SetBackgroundMode()
     */
@@ -1037,10 +1037,15 @@ class wxDC : public wxObject
     const wxColour& GetTextForeground() const;
 
     /**
-        @a mode may be one of @c wxPENSTYLE_SOLID and @c wxPENSTYLE_TRANSPARENT.
+        Change the current background mode.
 
         This setting determines whether text will be drawn with a background
         colour or not.
+
+        Default is @c wxBRUSHSTYLE_TRANSPARENT, i.e. text background is not
+        drawn.
+
+        @param mode one of @c wxBRUSHSTYLE_SOLID and @c wxBRUSHSTYLE_TRANSPARENT.
     */
     void SetBackgroundMode(int mode);
 
@@ -1980,7 +1985,8 @@ class wxDCBgModeChanger
         @param dc
             The DC where the mode must be temporary set.
         @param mode
-            The background mode to set.
+            The background mode to set, one of @c wxBRUSHSTYLE_SOLID or @c
+            wxBRUSHSTYLE_TRANSPARENT.
     */
     wxDCBgModeChanger(wxDC& dc, int mode);