Update disposal comments in PdfDocumentCanvas to clarify lifetime management for SemanticTag and WriteStream

Source/QuestPDF/Drawing/DocumentCanvases/PdfDocumentCanvas.cs

@@ -110,10 +110,10 @@ namespace QuestPDF.Drawing.DocumentCanvases
             Document?.Dispose();
             CurrentPageCanvas?.Dispose();
             DrawingCanvas?.Dispose();
-            
-            // don't dispose SemanticTag - its lifetime is managed externally
             SemanticTag?.Dispose();
             
+            // don't dispose WriteStream - its lifetime is managed externally
+            
             GC.SuppressFinalize(this);
         }