auth.texi, epa.texi, gnus.texi, info.texi, message.texi: Use @file for buffers, per the Texinfo manual
texi/ChangeLog: Fix previous commit
gnus-sum.el (gnus-summary-toggle-header): Display header attachment buttons

* gnus-sum.el (gnus-summary-toggle-header): Display header attachment
  buttons when toggling the header off.
* gnus.texi (Ma Gnus): Mention header attachment buttons.
tls.el (tls-program): Reflow docstring