add comment clarifying that `out is None` when verbose=True

IPython/nbconvert/post_processors/pdf.py

@@ -51,6 +51,8 b' class PDFPostProcessor(PostProcessorBase):'
                     out, err = p.communicate()
                     if p.returncode:
                         if self.verbose:
+                            # verbose means I didn't capture stdout with PIPE,
+                            # so it's already been displayed and `out` is None.
                             out = u''
                         else:
                             out = out.decode('utf-8', 'replace')