debug-on-error in init file

[Chong Yidong]

According to doc/lispref/debugging.texi:

    If your init file sets `debug-on-error', the effect may not last
  past the end of loading the init file.  (This is an undesirable
  byproduct of the code that implements the `--debug-init' command line
  option.)  The best way to make the init file set `debug-on-error'
  permanently is with `after-init-hook', like this:

      (add-hook 'after-init-hook
                (lambda () (setq debug-on-error t)))

This statement has been in the manual since before 1995.  However,
startup.el contains code that ensures debug-on-error is set correctly
(line 1106 and thereabouts).  According to vc-annotate, this code was
committed in 2001.

So, I think this is cruft.  Unless there are any objections, I will
delete it from the manual soon.