Add missing documentation for wxTreeCtrl GetSpacing / SetSpacing

Closes https://github.com/wxWidgets/wxWidgets/pull/1787

interface/wx/treectrl.h

@@ -506,6 +506,12 @@ public:
     */
     virtual unsigned int GetIndent() const;
 
+    /**
+        Returns the current tree control spacing.  This is the number of
+        horizontal pixels between the buttons and the state images.
+    */
+    unsigned int GetSpacing() const;
+
     /**
         Returns the background colour of the item.
     */
@@ -850,6 +856,13 @@ public:
     */
     virtual void SetIndent(unsigned int indent);
 
+    /**
+        Sets the spacing for the tree control. Spacing is the number of
+        horizontal pixels between the buttons and the state images.
+        This has no effect under wxMSW.
+    */
+    void SetSpacing(unsigned int spacing);
+
     /**
         Sets the colour of the item's background.
     */