Document wxPrintData::SetPaperSize().

Also remove strange (probably out of date?) comment from this method in the
header.

Closes #17035.

interface/wx/cmndata.h

@@ -384,9 +384,19 @@ public:
         Sets the paper id. This indicates the type of paper to be used. For a
         mapping between paper id, paper size and string name, see
         wxPrintPaperDatabase in @c "paper.h" (not yet documented).
+
+        @see SetPaperSize()
     */
     void SetPaperId(wxPaperSize paperId);
 
+    /**
+        Sets custom paper size.
+
+        This method can be used to set up custom paper size. When using a
+        standard size, prefer to use SetPaperId() instead.
+     */
+    void SetPaperSize(const wxSize& size);
+
     /**
         Sets the printer name. This can be the empty string to indicate that
         the default printer should be used.